Red Team are in a classic vert stack. Blue Team want to help the downfield defense.
Blue 2 could leave the handler, but that opens up the break side dishy.
Blue 3 could poach into the lane, but that opens up a short break throw to the anchor.
The 'dead' space is where Red 4, 5, 6 are standing, in the middle of the stack. Blue Team don't really need to guard that area so tightly.
Blue 7 already has the deep space covered.
Blue 5 can look to help against any in-cuts onto the open side.
Of course, this leaves the break side space open for Red 5.
So Blue 4 can help over in that direction, knowing that Blue 5 can pick up Red 4 if the make an in-cut.
Blue 5 can pick-up any in-cuts here.
And Blue 4 can pick-up any break-side cuts.
Blue 6 can stay on their man, but knows that Blue 7 is helping deep.
With Blue 6 already defending this in-cut, Blue 5 doesn't need to help.
As long as the Blue Team communicate and play smart, they can keep switching to pick up each in-, break-, or deep- cut.
If a play does develop, Blue Team must recover and think about their man first. They aren't playing Zone, they still need to guard man-to-man.
Until the mark is ready and a stack is in place, don't risk leaving your man unguarded.
But if Red Team reset back to a Vertical Stack...
Then Blue Team can look to help in the same way.
Picking up in-cuts.
Or break cuts.
Or some combination.