A simple way to increase adductor flexibility while doing bound angle pose is to open your hip creases.
The hip creases (termed "the kua" in Chinese) are the two lines on either side of the body that separate the lower belly from each of the inner thighs. The actual crease is caused by a ligament called the inguinal ligament. This ligament attaches at its upper end to the ASIC, the "point of the hip crest" and at its lower end to the pubic synthesis aka pubic bone.
It's actually formed by layers of the various abdominal muscles and actually serves as one of the points of attachment for some of these muscles. As a result it can be acted on by these muscles.
Because it attaches from one point of each innominate bone (hip bone) to another point, it creates a defined passageway through which muscles (and/or there connective tissue end points) pass. The two muscles are the iliacus and the psoas.
Iliacus attaches to the inner front surface of the innominate. It passes forwards and down to over the lip of the hip bone between the inguinal ligament and the hip bone and from there folds back to attach to the top of the inner surface of the femur, just below its neck at a prominent point called the lesser trochanter.
The psoas attaches to the bodies and transverse processes of the lumbar vertebrae as well as the intervening discs. It passes forwards and down and like the iliacus, folds around the lip of the hip bone, passing between it and the inguinal ligament, to attach at the greater trochanter.
Note that both the ASIC and the pubic bone are prominent corner points of the hip bone and the inguinal ligament attaches to them both. In addition, the ASIC has three long thigh muscles that attach at it or near it. Meanwhile the pubic bone has one long thigh muscle that attaches near it.
The long thigh muscles are muscles that attach near the corner points of the hip bone to the inner or outer aspect of the lower leg bones. These muscles thus work on both the hip and the knee and help to control hip bone positioning and/or knee rotation.
Other "deep hip" muscles also attach near enough to the ASIC and pubic bone that they can affect of be affected by the deliberate control of these points.
So what does it mean to open the hip crease? There are various ways to open and close the hip crease. In bound angle, you can get an affect similiar to opening the hip crease by pressing the knees down. However, if you instead focus on creating an opening (or deep stretching) sensation at the hip hip crease you may find that your glutes activate. You'll at the very least get some sort of muscle activation at the back of the hip.
Note, if you are already extremely open in this pose, such that your knees touch the floor easily, then you may find this action difficult to create. However, if you are only a little bit "tight" or "inflexible" then you may find that you can create an opening sensation at the hip crease.
The opening sensation comes from connective tissue structures being stretched. At the same time you'll get muscle activation from opposing muscles that actually drive the stretch.
One of the advantages of this action, versus say simply pressing the knees down, is that you are using muscles that work directly on the hip joints.
Note that the actual sensation of "opening" may or may not occur at the hip crease.
For myself, when teaching classes, I've found that the hip crease is a useful reference point. I use relatively simple exercises to help my students get a feel for opening their hip crease and closing it. As an example, to open the hip crease a simple exercise is to externally rotate the thigh. To close it, you internally rotate the thigh.
The nice thing about the hip crease is that it is relatively easy to connect to. Most students recognize where and what it is and so can use it both to drive actions of the hip hip joint and to feel the results of those actions on the hip joint.
For any muscular activation that I teach there can be multiple signals or "sensations" that are generated. With opening the hip crease the initial focus can be on noticing the hip crease and opening it (by adducting the hips). You can then notice the opening or stretching sensation that occurs. However, because muscle power is being used to drive this action you can also notice the sensations that are generated by those muscles. In bound angle this can be the gluteus maximus, but also other external hip rotator and hip abductor muscles can also be active.
The important point here, at least in the beginning, is not which muscles are active (though with deeper exploration this could be figured out). It's noticing the change in sensation as muscles activate and relax.
Note that in poses like side bends, it's pretty easy to feel one side of the body opening and the other closing.
To deepen the side bend one action that I like is to push outwards from the center of the bend. So for example, in a side bend you could push the hips outwards, towards the outside of the bend. Likewise you could push the bottom part of the ribcage in the same direction, out of the bend.
With opening the hips as in bound angle pose you could try a similiar action. In this case, the outside of the bend is towards the front of the hips. (That's where the "opening" or the "stretch" is occurring.) So as you open your hips creases, imagine pushing the top of your thigh bones upwards, towards the hip creases.
One way that you could think of this is as centrating, or helping to centrate the hip joint. Rather than the ball of the femur pushing towards the bottom of the hip socket, by creating an upwards push, you help to alleviate this tendency.
I should mention here that balance is required. Push too much and you go too far in the other direction.
As a final note, one way to close the hip creases and thus counterpose the stretching from bound angle is to do a wide leg forward bend.
Here, as you tip your pelvis forwards, the focus can be on closing your hip creases.
For more instruction on opening and closing the hip creases in different ways (and adjusting these hip actions) check out Lessons in muscle control for your hip crease, buttocks and inner thighs.
This course includes 5 workshop style routines with specifici exercises for feeling and controlling your hip creases as well as the muscles that affect your hip creases.
Each routine focuses on a slightly different aspect of hip crease control as well as including how to activate the hip joint muscles without using the hip crease as a reference.
The routines and the exercises are based on routines and exercises taught in my live classes with students of a wide range of levels. I've included the exerises (and adjustments) used for students who had particular difficulty feeling these actions.