Stand beside a box with your feet about hip-width apart, the box at your side rather than in front of you. Bend your knees and swing your arms back, then drive sideways and up, bringing both feet onto the box together. Land softly with bent knees, absorbing the impact rather than landing stiff-legged, and make sure you are fully balanced before stepping back down. The sideways direction asks your hips and ankles to control force from an angle they do not usually see in a straight jump, so keep the movement compact rather than trying to cover a lot of distance corner to corner.