This is kind of hard to explain. Plyo jacks are in a lot of videos, like Intense Moves with Gin Miller and many Cathe videos.
It's like a jumping jack but you kind of add a squat when you jump out and a squat when you bring your legs back in. It's not as fast as a jumping jack since you have to pause and add that little squat, but it's an intense move that really works the legs.