Throwing both arms back and up on take-off instead of taking just one along. In young female volleyball players the arm swing raised jump height by roughly 15–19%.
This is the cheapest centimetre you will ever get at the net. The arm swing raises torque and angular velocity in the legs, lifts your centre of mass and with it your take-off velocity — in a study on young female volleyball players that came to roughly 15 to 19 per cent more jump height, without a gram of extra strength. The typical error: the hitting arm goes back during the approach while the other one simply hangs there, so half the lever is missing. Done properly, both arms travel back and up together during the final step and are thrown upward as you leave the ground. What tends to get missed is timing — the swing has to match the plant step. Too early and the arms are already down when the legs push; too late and you are in the air before they help. Film yourself from the side: on video the fault is obvious, by feel it almost never is.