Presumably the defense will line up with a man marking each of our men in the bunch. They will likely also have a near and far post man. And one guy in the box between the kicker and the goal or short pass. The back on that side starts far enough out that they are unguarded, seemingly not a threat. They then make a run toward the box just before the kick...hopefully drawing the man in the box to them so the CM can have a better cross target in the box. If nobody moves to guard the back, the pass goes to the back who takes a shot on goal. The bunch still runs in to create confusion and also for the ricochet.