Professional introduction! Is it effective to fold your legs when running? It's kind of like kicking back.
Folding your legs means lifting your legs and pressing them down. Folding your legs helps the next step and makes your first and second steps go faster. This is the step frequency. Folding your legs can make your steps bigger. This is the step size. If you have step size and step frequency, it is impossible to run fast, so the running legs must be folded.