For the longest time I didn't pay pectoralis minor much heed, focusing insted on learning to feel and control serratus anterior since I felt it was of prime importance in stabilizing the scapula (shoulder blade.)
However, being as I walk around with a back pack quite frequently, and I ride a motorcycle, both activities which tend to encourage a "shoulders forwards" posture, I finally figured out that perhaps I'd better learn a bit more about the pectoralis minor.
First of all it attaches to a finger of bone near the top of the shoulderblade. If you feel the top of the opposite shoulder with one hand you'll notice a bony point at the top of the shoulder. That's the acromion process, the point to which the collar bone attaches to the shoulderblade. The coracoid process (coracoid means "crow") sticks out just undereath. The biceps long head attaches to it as well as the coracobrachialis muscle. And the pectoralis minor attaches there also.
From the coracoid process the pec minor reaches downwards, forwards and inwards to attach to the 3rd, 4th and 5th ribs. (Ribs are numbered downwards from top to bottom with the top rib being number 1 and the lowest rib numbered 12.)
Pectoralis minor lies beneath pectoralis major (the "pecs").
When activated, it pulls the top of the scapula forward, inwards and down, as if to flip the shoulderblade over the ribcage.
If you are used to wearing a back pack it might be this muscle that you are using to push forwards against the straps of your back pack.
Working in combination with serratus anterior, pec minor can be used to protract the shoulderblade, meaning it helps to pull the shoulder blade outwards, away from the spine, and around the sides of the ricage towards the front.
Working with levator scapulae and rhomboids, it can be used to downward rotate the scapula. In this case the inner border of the shoulder blade moves upwards while the outer edge (including the shoulder socket) moves downwards.
Working with the lower trapezius, it can be used to depress the scapula.
As mentioned, it can be used to "flip" the shoulder blade over the shoulder.
One of the places this action can also be useful is in the Prasarita Padotanasana c arm position. Doing this while upright it can be used to lift the arms extra high. When bend forwards you might be able to used this action to help get your hands to the floor. At the very least you can use it to help increase the angle between your back and your arms.
It may be even more helpful when you do this position with arms internally rotated and palms facing outwards.
It can also be useful when trying to getting the hands into the prayer behind the back position used in parsvottanasana.
Flipping can be very useful in binding yoga poses like Ardha Matsendrasana and Armpit pose when clasping the hands behind the back. For the binding arm (the left arm in both pictures below), use pec minor to help rotate the arm inwards.
And it can be useful in Cow Face pose, particular for setting up the lower arm.
To deliberately activate this muscle first lift and open your chest. Then, focusing on the bony peak of the shoulder. Draw this point downwards and inwards towards the middle of your chest. You can rotate your upper arm inwards and reach it backwards, outwards and upwards at the same time. Your pectoralis major will probably also activate but that's fine.
And after trying this action with chest lifted, try it again with your upper back bend forwards. Try contracting your abs.
The feeling is like you are pulling the peak of your shoulder forwards, downwards and inwards towards the bottom of your sternum. Try to activate as you inhale and then relax while exhaling.
Once you have a feel for activating the muscle, try to activate it while doing the prasaritta padotanasana c arm position and then while trying to get into the prayer behind the back position.
(You can always hold on to a towel if you can't clasp your hands behind your back.)
And experiment with its use to see if it helps (or hinders) binding in ardha matsyandrasana.
To stretch pec minor I like to reach the arms out to the sides, spread the shoulder blades and then externall rotate the forearms and upper arms while at the same time moving the top of the shoulders back.
You can also try the same thing with the arms angled downwards at about 45 degrees and slightly back. Make sure that you open your chest at the same time.
Another good stretching position is table top pose. Instead of using the shoulders to push your ribcage up, let your ribcage hang from your shoulders. Try moving your shoulders back and forwards slightly to vary the stretch. In the picture below my legs are crossed. You can practice relaxing your shoulders in this position so that you chest sinks. Then try to do the same but with knees bent and pelvis lifted.
Why improve muscle control?
Muscle control not only helps you to control your body, it also helps you to feel it.
Muscle activation creates the tension that not only moves your body, but helps you to "sense" it.
With better muscle control you can use your body with less effort and make it easier to balance, improve flexibility and deal with pain and poor posture.