This exercise is based on the standard split squat. Just add a couple of boxes to elevate both feet. By standing on boxes you'll allow for greater range of motion and that's precisely the goal of the exercise.
The athlete pulls himself deeper into the hole with each rep (3-5 second holds). This not only hammers away at those pesky tight hip flexors, it also develops a great amount of starting strength by working the athlete in his greatest (and weakest) joint angles.
Pull yourself into a deep position by pulling with the front hamstring. Flex and extend the rear leg glute as hard as possible. Always pull deeper on subsequent reps.
If you're looking for some serious upper leg development, then try a few heavy sets of these and feel sore in some places you didn't even know existed.