In a maul a defender can rip the ball from the ball carrier or lock it up so that the carrier can't get it back to their team but cannot pull the player down. If the maul doesn't go forward and the ball carrier can't get the ball out it is a turnover so defenders try to wrap up the ball so it can't come out. As soon as the ref has called maul the ball carrier needs to look to go to ground if his maul is not going to move. Once a knee touches the ground it is a tackle so everyone needs to release the ball carrier then it becomes a ruck.
The tackler needs to get the ball carrier to the ground initially in the tackle so that a maul can't be formed, but difficult to achieve in lineout situations.