I’m not an advocate of stretching in general. Stretching is primarily important for athletes who participate in sport where flexibility is essential. Beyond the sport perspective one should only stretch if it limits them from doing something in their daily lives such as reaching for an object on the floor. Stretching can Infact be dangerous since it can increase joint laxity. This is apparent in shoulder injuries with those who have hyper flexible shoulders.
However, for your question, stretching slowly is important because stretching fast can cause injury. Muscle is like a rubber band. If you pull fast it can tear and snap. If you pull slowly it gradually lengthen and not tear. All muscle that is stretched will recoil back to its original length. But consistent chronic stretching will provide the muscle the ability to lengthen to certain limits with ease. Bouncing is the same idea as a rapid stretch and thus dangerous.
