Stretching that is characterized by a "bouncing" action is called
Ballistic stretching.
It helps efficiently to increase your range of motion but is known to create an injury. That's why they tell you NOT to do this specific exercise, unless you have a professional trainer with you that can help you.
Anyways, Good Luck! :)
I’m fairly certain it would be cocaine, since it’s much more common and is more of the stimulant.
The goal of flexibility is to allow your body to have more range of motion in your joints