Adductor magnus is one of the few "internal" rotators of the hip. (There seem to be lots more external hip rotators than internal hip rotators!)
It has two parts, one of which attaches from the sitting bone to a point on the inside of the thigh bone, just above the knee.
When this part of the adductor magnus activates, it creates a rearwards pull on the inside of the thigh helping to rotate it inwards, or helping it to resist external rotation.
A broader portion of the adductor magnus attaches along the entire length of the bottom of the pelvis (to the ischiopubo ramus which connects each sitting bone to the pubic bone) and from there to most of the length of the back of the thigh bone.
This portion of the adductor magnus can be used to pull the thigh inwards (adduction) or resist it being pulled outwards (abduction)
While standing with legs straight, one way to deliberately activate adductor magnus is to press the inner thighs back. You should be able to notice a line of tension from the sitting bones to the inside of the thigh just above the knee as you do this.
When working to prevent external rotation, the adductor magnus could be working against external rotators or it may be working against the weight of the thigh itself.
As an example of the latter case, with the leg lifted rearwards such as in extended cat pose, you could focus on pulling upwards on the inner thigh just above the knee while allowing the mass of the outer thigh to hang down.
To use the adductor magnus to extend the hip, simply pull the inner thigh (just above the knee) higher while continuing to allow the outer thigh to "hang" down.
The Adductor Magnus is often likened to being an hamstring because like the hamstring muscles it runs from the sitting bones, running down the back of the thigh to attach to the inside of the femur, just above the knee.
The difference is that the hamstrings attach below the knee to the tibia and fibula so that they work on both the knee joint and the hip joint. Instead of operating on two joints the Adductor Magnus only operates on one, the hip joint.
In a belly facing down yoga pose like extended cat pose, you can use the adductor magnus to lift the leg and keep the knee pointing down with minimum effort.
It can feel like the leg is hanging from the inner knee.
Rather than using the gluteus maximus to lift the leg, or rather than just using the gluteus maximus, you can use adductor magnus to keep the leg suspended and even to lift it higher.
And so that the knee points down naturally, due to the weight of the leg, you can pull up on the inner knee and thigh to activate adductor magnus while at the same time letting the weight of the outer thigh hang down.
It may be that because the leg is horizontal the adductor magnus is better suited because it's point of attachment is closer to the legs center of gravity than say the gluteus maximus.
As a result I find it easier to lift the leg (while still keeping the knee pointing down) by focusing on pulling up with the inner thigh.
Initially, reach the leg straight back so that it is horizontal.
As you get better at using the long head of the adductor magnus you can try to lift your leg higher. To make this action easier, try tilting the pelvis forwards first so that you increase the lumbar lordosis.
You could tilt your pelvis forwards first and then lift the leg higher or lift the leg higher as you tilt your pelvis forwards, as if moving your leg and pelvis as one unit.
One way that you could justify tilting your pelvis forwards first is that it then gives your thigh room at the hip joint to lift higher into hip extension.
Whether your aim is to keep the leg horizontal in extended cat pose or to lift it higher into extension, one idea that is important in any deliberate muscle activation is giving the muscle you are activating a stable foundation from which to act. Since the adductor magnus attaches between the thigh and the pelvis you could either somehow stabilize the thigh (by making the knee stable) or stabilize the pelvis.
If tilting the pelvis forwards, you may be already stabilizing the pelvis, particularly if you work at keeping it tilted forwards while lifting your leg higher. One way to stabilize it further in cat pose is to create a downwards pull on the points of the hip bones (the ASICs). You can learn more exercises for stabilizing the pelvis with Improving Hip Joint Control
In either of these two poses, with your hips lifted, focus on pressing the inner thighs down to activate your adductor magnus. Keep your pelvis lifted as you press your inner thighs down.
Because the long head of the adductor magnus is an internal rotator, activating it in bridge and wheel pose may help you to keep your knees pointing in the same direction as you toes. But prior to lifting up, I'd suggest that you set your feet up so that they are parallel.
One other backbending posture where the Adductor Magnus may be useful is in Camel Pose.
In extended cat pose you lift one leg and move the leg backwards with respect to the pelvis (or actually, upwards.)
In wheel and bridge you use the legs to push the pelvis upwards. Because the feet are in the floor in both of these poses, the result is that the pelvis moves up relative to the legs.
For camel pose you can start from a kneeling position.
In camel pose, you may find it comfortable to have your shins about hip width (or maybe even shoulder width) apart.
Use a position that is comfortable given what you are trying to do.
With your hands on the floor, push your hips forwards and up. Once your hips are lifted, you can recruit adductor magnus by pushing rearwards on your inner thighs while continuing to push forwards through your pelvis.
Although I am not expert at dropping back (it's one of those actions that I had for a while and then didn't do for awhile and then got too scared to actually do the drop back) when I was doing drop backs I did them with my feet turned out alot. I'd collapse into my arches which made it easier to push my knees forwards so that then I could get my center over my feet. This made it easier to drop back and land on my hands and also to pull back up to standing.
I recently tried to do a standing back bend (without actually dropping back) and found that pushing the inner thighs back while pushing my pelvis forwards allowed me to bend back fairly deeply. However another component of this was pulling the sitting bones inwards and the tailbone forwards towards the pubic bone.
In view of what I recently wrote about the SI Joint and stabilizing it the opposite action, moving the tailbone rearwards and sitting bones outwards, might actually be more appropriate.
And that is actually the bigger point of all articles that I write. Experiment, and while experimenting see for yourself what works and doesn't work. And continue to experiment because what doesn't work one day may actually work very well on another day.
Although I've talked mainly about activating the adductor magnus by pushing the inner thigh back (and it's my assumption that its the adductor magnus that is activated by doing this "action") There are also times where I've experimented with pushing the inner thighs forwards.
While this may not be the adductor magnus that is activating, it's good to be aware of this potential action. I've found that if I'm not sure what to do, I try both and see which works best.
One other area where pressing the inner thigh back or forwards may be helpful is when doing one legged balancing poses. I've found that when standing on one leg pressing the inner thigh forwards (half moon pose) or backwards (warrior 3, tree pose, eagle?) helps to make the standing leg more stable. And it also helps when standing up from triangle pose (press the inner thigh of the front leg "forwards.")
In the hip joint article I talked about creating space in the hip joint using the obturators and the gemellus, all of which are external rotators.
I also suggested that this external rotation is probably counted by the Adductor Magnus to keep the thigh from rotating. (possible the TFL and gluteus minimus as well.)
It may be that if you press your inner thigh back a natural response is for the obturators and gemellii to activate and so activating the adductor magnus may automatically help to create space in the hip.
But if it doesn't, you could experiment with activating adductor magnus (by pressing down with the inner thigh, or back if you are standing) while at the same time making your thigh feel long.
Sitting upright with legs straight ahead you could focus on pushing the inner thighs forwards and down to activate both sets of muscles. You can try using this action in any forward bending hamstring stretch.
So that you can lengthen your hamstrings with greater ease you can also experiment with engaging your buttocks at the same time so that they take the strain of supporting your upper body. You can then gradually tilt the pelvis forwards using the gluteus maximus as the controlling muscle.
Note that this can feel like a "spreading" or in Richard Freemans' words a "flowering or blossoming" of the buttocks. (actually those aren't his words, but I'm sure he said something like that, while at the same time reminding me of a bushy eyebrowed agent smith from the matrix.)