We know that  in general a push or pull is nothing but force. here the force acts as  frictional force. so, this is the resistance to the movement of the body. when we applied force from front the friction force acts from backwards which helps in the swinging.and due to the pass of time the force applied will be decreased so we used to apply force after every sometime so this is reason i hope. 

tq for asking these type of question to us