Quote: Richie "Well if you're going to look to put pressure on the hooker, then your marker has to go to one side and go forwards, opening up space both to the other side for the hooker, and back behind the PTB for a runner.'"
You also run the risk if you get out too quick of being pulled for not being square.
Splitting the markers enables you to effectively keep your defence in a line, with the markers covering the gap between the two A defenders either side of the PTB. If you go off like a headless chicken chasing the ball it's going to leave gaps elsewhere.