Standing still position requires very little muscle use. Frame supports the body. Quadriceps muscles lock knee as the body is shifted forward. The shifting is done by the weight being placed ahead of the grounded foot by releasing the pressure of the toe, and swinging the raised foot forward. No power to vault is exerted by gravity.
Air time is the slowdown time in the stride cycle.
It shouldn't be made longer than necessary to comfortably exchange the feet. Get off the vaulting leg at a low point to skim the foot low to the ground. Keep the toe raised to avoid tripping. Land flat on the foot, and use the ankle for rotation. It gets higher and longer with faster speed.