Simply put, kids combine the technical analytical instruction with their desire to be comfortable (which is nothing else than the body’s signal of telling you what is a natural way of generating force), while adults do everything in their power to perform the movement “correctly,” even when it doesn’t feel comfortable and it doesn’t produce any natural force.
Many players around the world struggle to get their body weight going into the serve and don’t use their back leg to help them generate extra power on the serve but also balance the body once they land. By kicking back your right leg (left leg if left handed) when you land onto your front foot, you are helping the body to balance and that extra push from the ground during the upward phase on the swing will help you generate extra power on the tennis serve.
Tennis is a racket sport that can be played individually against a single opponent (singles) or between two teams of two players each (doubles). Each player uses a tennis racket that is strung with cord to strike a hollow rubber ball covered with felt over or around a net and into the opponent's court. The object of the game is to maneuver the ball in such a way that the opponent is not able to play a valid return. The player who is unable to return the ball will not gain a point, while the opposite player will.