The vastus medialis or VMO is the tear-drop shaped muscle on the inside of the knee. Unless specifically targeted in training, it commonly falls behind in strength compared to the well-developed lateral (outside) muscles.
This exercise will keep the VMO from lagging behind. It's something all athletes should be doing at some point during their workouts. Not doing so leaves them vulnerable to knee problems and future injury, like ACL tears.
For this intensity technique, do the concentric part of the exercise on your own, then have your training partner push down during the negative as you resist.