The first key concept is RELAX into the stretch. Pavel Tsatsouline literally wrote the book on it.
I first took note of this phenomenon when trying not to squeal during a deep tissue massage. The pressure on my hip flexors and adductors were brutal. If I flexed my stomach muscles, however, the elbow into my inner thigh became tolerable.
Why the abdominals? My guess is because it’s the congruence of most parts. It’s centrality is what makes it so effective. (I believe you should feel every movement in your abs, but that’s for another post). The torso is also home to to the thickest bands of tissue, whose elastic strength, when contracted, leaves room for slack in other bands.
I was recently working with a few gals on beginning 90/90 reaches for hip mobility. They were having quite a hard time with it, and their face and body language showed they were battling with the leans, trying to coerce their hips into submission. They even spoke of ‘back twinges’ on particular rotations. I cued them “contract the different parts of the stomach” in the sticking points. Abdominal activation should alleviate the drive through the back and allow the tautness in their hips to lessen. To everyone’s delight, within a few seconds, they gained an inch or two in depth, showed much less discomfort, and were breathing deeply.