D1 is marking the disc, with a force to the right side. Let's call that side away for this example. D2 is defending O2.
Because the force is away, D2 knows that the passes most likely to be completed are on the away/open side, so guarding that space, or the path to the space is key when O2 is in a mid-field position.
When O2 is in the mid-field, and D1 is forcing away, D2 staying close to O2 can put D2 at a positional disadvantage ...
... because when O2 cuts to the open space, there's a lot more room to move once O2 gets open. O2 makes a hard in-cut fake, then when D2 is fully committed ...
... plants and cuts upfield in the open space.
When D2 is in that space, D2 has more time and space to respond to O2's cut.
D2 doesn't have to bite as hard on a hard in-cut to make that pass difficult to make or of a negligible gain ...
... and when O2 looks to get open on the force side/open side/away side, D2 has a couple steps on the person and can close space pretty quickly.
And while it seems intuitive, it bears mentioning that when you know what force D1 is applying, D2 doesn't want to be on the break/closed/home side of that force ...
... because right from the start D2 is at a positional disadvantage.
When O2 is in a deeper position, D2 has to determine ahead of time what cut they won't contest - either the deep-cut (D2 starting behind O2) ...
...or the in-cut/under.
If D2 is contesting the in-cut, when their mark goes deep it's important to stay close ...
... but not too close. If D2 leaves too much open space on the open/away side - let's say 1/2 the field width or more - ...
... O2 only has to use a couple steps to get open and pull in that deep pass.
Maintaining positional advantage in the deep field is similar to the mid-field ...
... staying in the open space gives D2 time and space to react to O2's cut ...
... so the highest value cut (i.e., high percentage pass caught in the end-zone) is not an option.
BAM!