The last, long step of the spike approach: it brakes you and redirects your speed upward. Without it you jump from a standstill instead of from a run-up.
A spike approach runs over three or four steps, but only one of them decides your jump height: the last. It is deliberately long and braking, because its job is converting horizontal speed into vertical — and spike jump height depends precisely on how fast your centre of mass is travelling upward at take-off. The most common fault is an approach that stays comfortable throughout: you arrive under the ball cleanly enough, but you have nothing left to redirect. The second is a final step that is too short, which tips you forward into the net instead of up over it. Build speed across the steps and make the last one deliberately long. What tends to get missed: the plant step does not only brake, it also loads the arm swing. Both arms travel back and up as it lands — legs and arms are one movement, not two.