Stand with your feet together, elbows bent 90 degrees. Lunge forward with your right foot [A]. Jump straight up as you thrust your arms forward, elbows still bent. Switch legs in midair, like a scissor [B], and land in a lunge with your left leg forward [C]. Repeat, switching legs again. That's 1 rep; do 10.
To prevent injury, try to land as softly as possible.