Split Stance Kneeling Adductor Mobilizations are a staple here at Cressey Sports Performance within our warm-up routines. This mobilization drill is a great way to move through hip flexion on one leg while feeling a nice stretch throughout the adductors and hamstring muscles on the opposite leg. I love this drill as well as the position, and always try to find new ways to explore this set-up.
One of my favorite drills in this split-stance position are Hip Abduction End-Range Lift-Offs. These will challenge you in a way you've never been challenged by a hip mobilization drill before. When building tension throughout your entire body, you're forced to stabilize the entire hip joint in a new range of motion it's not adapted to yet. Over time with constant practice, this will help expand the hip's range of motion while preventing injury due to the newly acquired strength.
I should warn you now, these are not easy and will probably make you cramp the first time you try a rep. Good luck!