The external obliques and anchoring the front of the hip bones
The external obliques attach from the lower 8 ribs (ribs 5 through 12). The lower fibers extend downwards and and slightly forwards to attach to the hip crest. The upper fibers angle forwards and downwards and help to form the connective tissue sheath of the rectus abdominis. Additionally, the lower aponeurosis or "tendon like structure" of the external obliques muscle help to form the inguinal ligament which attaches between the ASICs (the points of your hip bones) and the pubic bone.
While the portion of the external obliques that attach to ribs 11 and 12 tend to attach to the crest of the hip bone, the middle fibers, from rib pairs 8, 9 and 10 may tend to more directly form the inguinal ligament.
Anchoring muscles that work on the hip and the knee
Assuming the front of the ribs are lifted, these fibers have enough length to effectively active and create an upwards pull on the inguinal ligament as well as the ASIC and the pubic bone. This in turn can help the hip bone act as an anchor for the gracilis, iliacus, sartorius, rectus femoris, gluteus minimus (and perhaps gluteus medialis), and the tensor fascia latae.
Some of these muscles (sartorius, gracilis, tensor fascia latea) can have an affect on knee stability, helping to stabilize the lower leg against rotation.While not involved in shin rotation, the rectus femoris may also help in stabilizing the knee (or at the very least is affected by shin rotational stability or its lack). Note that these muscles are all hip flexors. But they work on the knee joint as well.
How are the external obliques anchored
In general, muscles need a fixed end point or anchor point to function effectively. It's the rough equivalent of a crane extending out-riggers so that it can lift heavy loads. Or it's the equivalent of a vehicle with an winch being anchored so that it can help retrieve a vehicle (say a tank) that is stuck in mud.
The question can be asked, if the obliques can be used to help anchor the hip flexors and other muscles that attach to the hip bones, what then helps to anchor the upper end of the external obliques?
Lifting the chest and bending the spine backwards
Personally, when I use the external obliques to create an upwards pull on the inguinal ligament (and ASICs and/or pubic bone), my chest tends to lift automatically, as if to give the necessary length to the external obliques so that they can effectively active. In addition, what I tend to notice is a feeling of compression in the lower thoracic spine that seems to extend into the upper lumbar spine. The feeling is as if the vertebrae in that region are all stacked, and the compression happens along the axis of that stack.
Rather than an uncomfortable feeling, it actually feels quite satisfying, as if my back is braced to take the load that the obliques are exerting.
There is also the feeling of a slight back bend in the region.
While it is possible to lift the fronts of the ribs without bending the spine backwards, there tends to be a greater amount of lift when the spine bends backwards. Put in other words, when you lift the fronts of the ribs, the idea is to more them away from the front of the hip bones. You can get more distance between the hip bones and fronts of the ribs if you bend the lumbar and thoracic spine rearwards.
The iliocostalis, spinal erector muscles that act on the backs of the ribs
Viewed from the side, the lumbar spine tends to have a lordosis or backwards curve. The thoracic spine tends to have a kyphosis or forward curve. Where these two curves meet, an s-shaped (or taking a cue from violin makers, a cursive f-shaped curve) is formed.
There are spinal erector muscle fibers that attach from the back of the pelvis to the backs of the lower ribs (iliocostalis).
There are also spinal erector muscle fibers that attach closer to the midline of the spine that attach from the back of the sacrum and lower lumbar vertebrae to the backs of the lower thoracic vertebrae.
Even with the spine already bent backwards, these muscle fibers can continue to act from the back of the sacrum and pelvis to create a downwards pull on the backs of the lower thoracic vertebrae and the backs of the lower ribs.
This muscle activation is easily perceivable. You can feel it.
You can also learn to feel the change in curvature of the lower thoracic spine as it flattens or even reverses slightly so that the upper part of the aforementioned S-shape curve flattens.
Muscles need an opposing force to work against
A general principle of muscle control is that muscles need an opposing force to work against in order to activate. (Muscles create force when the activate, ergo, they need an opposing force to work against in order to activate.)
In a standing position, these spinal erectors can work against the weight of the ribcage and tension that results from stretching or lengthening the forward part of the abdominal wall (rectus abdominis).
Thus they can activate and create a backbending sensation with the abdominal muscles reasonably relaxed.
However, add tension to the external obliques, particularly those that attach to the inguinal ligaments, then the spinal erectors have to work against a greater force. And thus the sensation that the spinal muscles generate increases.
This is because now, instead of just working against the the weight of the ribs (and the tension from stretching the front of the abdominal wall), now the spinal erectors are also working against the downards directed force on the fronts of the ribs exerted by the external obliques.
This is particularly true for the spinal erectors that attach to and work on the backs of the ribs (portions of the iliocostalis)
Even though the obliques have more leverage via the ribs, the backwards curve of the spine in this region can help to compensate for that leverage so that the spinal erectors don't have to act with too much effort. In turn, the backwards curve of the spine in this region can mean that boney alignment, or vertebral stacking, can help resist the force created by these sets of opposing muscles.
What are the potential consequences, or rather, benefits?
Anchoring the sartorius, rectus femoris and tensor fascia latae
Basically, with the spinal erectors activated and creating a downwards pull on the backs of the ribs, the external obliques have a stabilized anchor at the fronts of the ribs. They can then effectively create an upwards pull on the inguinal ligament as well as the ASICs and pubic bone. This then provides an anchor for muscles such as the tensor fascia latae, rectus femoris, sartorius and gracilis, all of which act on the knee joint. (As mentioned, all of these are hip flexors, and apart from the rectus femoris, also shin rotators)
This isn't to say that you should always walk with lifted ribs in order to protect your knees. It's to note that in some circumstances, lifting your ribs may help your knees. And it's a fairly simple action to learn. And if you find your knees are giving you trouble and you notice you have a sunken chest, one possible quick fix is to lift your chest.
Reversing the curve of the lumbar (and lower thoracic) spine
What if we reverse the curve? What if, instead of curving the lower thoracic spine backwards we instead curve the upper lumbar spine forwards?
Any slack this induces in the aforementioned external oblique fibers could be reduced by activating the transverse abdominus. The transverse abdominis can pull inwards on the fibers of the external oblique, causing them to lengthen. This actually gives the external obliques a force to work against. And so not only does activating the transverse abdominis potentially lengthen the external obliques, so that they aren't too short to activate effectively, it can also provide a force for the muscle to work against. And this can be true whether or not the chest is lifted.
In either case, (whether lifting the chest using the spinal erectors so that the external obliques can create an upwards pull on the ASICs, or whether using the transverse abdominis to cause the external obliques to activate... or a combination of both) if the ASICs and pubic bone have an upwards pull, this can help to anchor the muscles that attach from these points so that they can work effectively on the knee joint.
A stabilized spine (that resists forward bending)
Note that whether using the spinal erectors (in particular the iliocostalis) along with the external obliques, or whether just focusing on transverse abdominis activation (where you may get external oblique and iliocostalis activation as a beneficial side affect), in either case you end up with a stabilized or "controlled" spine. And that may help you when dealing with some types of knee problems or knee pain.
And as mentioned in the introduction, if you find you get knee pain, notice the mood you are in when you get it. And notice the affect your mood has on your posture. Has your chest sunk down? When you notice things like this, you can take action and "adjust" your posture to see if that helps alleviate your knee pain. It may also go some way to helping you alleviate your mood.
For exercises to help you better feel and control the parts of your spine, including your ribcage and hip bones, check out the Muscle control for the spine, hip bones and ribcage course.
It's designed to help you get a better feel for your ribcage, hip bones and spine.
Hip flexor control
For exercises that focus on helping you to get a better feel for and control of the hip flexors check out the Hip flexor muscle control course.
This course focuses on the aforementioned hip flexors (tensor fascia latea, rectus femoris and sartorius) and how to anchor them as well as activate them and feel them.
Hamstring (and glute) control
For a course that teaches you to feel and control the muscles that act from (and work on) the other corner points of the hip bones check out the Hamstring muscle control course.
This course teaches you to anchor and to feel and control your hamstrings as well as your glutes.
Three course package
All three course can be bought together as a package: Bundle: Muscle control for spine, hip flexors and hamstrings.