When most people jump, they have a dominant leg.  When you do two legged exercises, the dominant leg will do most of the work, leaving the weaker leg to miss out on developing as much.  Single leg exercises isolate the weaker leg.  As the weaker leg catches up to the dominant leg in performance, the result is a nice benefit to your vertical jump.  You will find single leg exercises in the vert builder and vert builder pro programs on our site.  Check them out.