Lower Your Chest to Raise Your Knees
Mechanism / Why It Happens
Action and reaction. If you keep lifting your upper body while trying to jump, the reaction force pushes your legs downward.
Think about a basketball player reaching as high as possible. To extend their hands upward, their legs naturally move downward. The same principle applies to an ollie. While lifting your upper body is important, if your chest is still rising at the peak of the ollie, the reaction causes your legs to move downward.

What to Do
At the peak of the ollie, while your center of mass is still rising, drive your upper body downward. Use the reaction force to lift your knees.
Now think about a hurdler. To clear a high hurdle, they drive their upper body downward while liftin