Defenders must be on their feet as much as possible. If you are on the ground, you are out of the game. The next worst position to be in is turned sideways, whether slowly trying to get into position or turned to chase someone who isn’t your responsibility. An effective defensive line is one that is spread across the width of the pitch and with hips and shoulders square with the goal line. This position allows defenders to come forward to take away space and change direction together as needed.
Defenders in the front line should stand with the inside foot up. This prevents turning hips inward, forcing them to stay square and look inwards only with the head. When the ball emerges, the first step is forward, rather than a wasted step getting square.
Even in this position, defenders can hand over the initiative to attacking players by being flat footed. If they stand still and fail to come forward, attackers can aim for spaces or attempt to run them over at will. Even if a defender does come forward, but drops his/her butt and stops dead to prepare for a tackle, attackers can again take the advantage beat him/her.
It is important, then, for defenders to come forward once the ball is out and stay connected with each other. Some teams do an excellent job of stopping attackers well behind the gainline (or forcing unwanted kicks) by sprinting forward. There is a risk, however: a fast, long-striding defender is more susceptible to being stepped or can otherwise compromise the integrity of the defensive line.
Some teams do well with a slower, more calculated forward pursuit. This can be detrimental to an attacking team if they perform complicated moves that end up having dummy runners in front of ball carriers. This can allow the defending unit a chance to isolate the strike runner or turn up the tempo to smash a ball carrier well behind the gain line. Even if it does not provide the defending unit a clear advantage, coming forward as slow as the slowest player ensures the integrity of their line is maintained with no gaps.