Ballistic stretching is a form of passive stretching or dynamic stretching in a bouncing motion.
According to the American Academy of Orthopedic Surgeons, "[d]o not bounce your stretches. Ballistic (bouncy) stretching can cause injury."
Ballistic stretches force the limb into an extended range of motion when the muscle has not relaxed enough to enter it. It involves fast "bouncing" movements where a double bounce is performed at the end range of movement.
This type of stretching can cause muscles to get tight, and can cause serious injury. It has been replaced with static stretching.