This is one combination of two exercises that are great for developing power through the trunk and legs. Starting from a sit-up and going into any number of other exercise movements is a great way to increase the effectiveness of your workout.
There are two versions demonstrated here:
From the floor and from a low bench or step. From a bench or step is a great way to start doing these. Once they get too easy you can progress to the version starting from the floor.
Sit-Up Jump-Ups from a low bench/step.
Sit-Up Jump-Ups from the Floor.
Again, Sit-Up Jump-Ups from the Floor.
Have Fun!