Hyperextended Jumps are a result of a combination of characteristics:
First the athlete must be dynamically flexible enough. This means they must be able to lift their legs up on their own. Doing straddle lifts, pike lifts,hurdler lifts, and leg lifts will help this. Isometric legs lifts will also increase the strength of your hip flexors. Stretching your hamstrings will help with flexibility. Just be sure to condition the hamstrings also because strength yields more allowed flexibility. Always have them point toes backward.
Secondly, make sure the athletes core is very strong (plank sequences). It must be strong for them to roll their hips.
Thirdly, the athlete must have a strong push off the ground. Make sure they do jumping exercises as well as punching (plyos). You have to overcome gravity to lift yourself off the ground so you must be strong. Also train the athletic stance as it is the physiologically strongest loading of energy for explosion off the floor.
Fourthly, the arms must be strong to help lift the body upwards as they swing into the jump position.