Why is ballistic stretching no longer recommended for preparing the body for exercise?
It is still recommended and it is okay to stretch this way.
Ballistic stretching involves bouncing the body while stretching a muscle group past its current flexibility. For example, instead of reaching slowly and gradually extending your body towards your toes in a straddle stretch, you would bounce 4-5 times towards your toes. This can result in strained or torn muscles or ligaments.