most likely be space around or behind this one defender for the attackers to go at. if the whole defensive unit shifts accordingly, they don’t allow that space to be created (instead, the space is created on the far side but since most players can’t accurately and consistently play a ball 60-70 yards in the air it’s not dangerous). by having all of the defensive players stay “in touch” with each other as opposed to getting spread out it makes it harder for the attacking team to create dangerous situations.
the same thing is true forward and backward. if the attacking team plays a ball backwards and one or two defensive 