Stand on the box close to its front edge facing a vertical jump flag. Step off the box landing on both feet under the flag.
Do not initiate a rhythm of landing (1 – jump down, 2 – lend...). The landing should be a “surprise”, causing an explosive jump up. Some athletes stand on the heel on the edge of the box when the other foot is moved forward for the step, and let the heel slowly slide down off the box, so that the exact moment of falling down is somewhat unpredictable.
While falling down, prepare for landing by flexing at the knees and hips. Cock the elbows back and lock the feet into toes-up position.
Immediately upon landing, not after, initiate the jumping phase by thrusting the arms upward and extending the body for as much height as possible. Jump off the ground as fast as possible, with minimal ground contact.
Reach up with one or both hands and hit the highest flag possible.