The timing of putting the ball into the scrum is very important when on attack. The ball can be put in when the referee calls ‘engage’ or when the hooker signals to the scrumhalf.
This signal in most cases is a tap on the props shoulder, next to the scrumhalf. A tap call is recommended if the opposition is pushing early in the scrum. Teams are not allowed to push before the ball is in and this may result in a penalty.
The scrumhalf is supposed to communicate with the hooker to make sure that the required hook is presented. A small or shallow hook of the ball may leave it lying right at the base of the scrum, making it very hard for the scrumhalf to feed the backline