Defending behind the front line requires the Back Three – the full back and both wings – to work together. It is often said they cover each other like a pendulum swinging, though it might be simpler to think of all three of them being connected by a strong rubber band.
Typically, the two wings hang back a few metres from the main defensive line to discourage kicking (a totally flat defence leaves the full back to cover the entire back field solo). The full back typically shadows the ball carrier, deep enough to cover a potential kick. When the ball moves toward the wing, the winger has to come forward and link up with the main defensive line. This leaves a hole behind perfect for a kick, especially if the full back is still in a central position.
Within this scenario, then, the full back typically slides behind the outside backs. This would leave a large hole in the middle of the field, so the off-side winger has to come across to cover this space. This is how the Back Three work as a ‘pendulum’, swinging across the back field to ensure there are no easy gaps to run or kick into.