A standard vert stack vs a home force (the orange triangle represents our gear, thus home. The opposite side would be "away")
The Red 1 (R1) is standing on the left hip of Blue 1 (B1), forcing them to throw towards "home". Ideally, R1 would NOT allow B1 to throw anything to the "away" side (breakside). If you allow your mark to make a break throw, shout "BREAK" or "BROKEN"
If we do get broken (it's going to happen, it's ok), we have to make sure to keep the force or everything will go to shit. Note that R2 came back around to force B2 home. Everything resets around that.
Resetting
The idea behind forcing to one side or the other isn't necessarily to make it a difficult throw, it's to make it easier for the other defenders to cover their man.
B2 makes an upfield cut, because R2 is on the force (home, in this instance) side, the throw must be around or over R2.
If B2 cuts back towards the disc, R2 is already in position to keep their body between their man and the disc
There is the danger of being beat deep, so if you know that your man is fast enough to do that, give them some cushion and let them have the underneath short catch
Resetting
To avoid getting beat deep, just play a few feet/yards off your man.
They will probably come underneath, but that's better than getting beat deep. Once the field shortens as they advance we can tighten up.
Just make sure to reestablish the force
Defending points of emphasis: 1. Keep the force - someone will call it at the beginning of the point. Home = sideline where our stuff is. Away = sideline where our stuff is not.
2. Whenever your mark throws the disc, yell "UP" (players on sideline should do this also)
3. If you get broken, yell "BROKEN", and reestablish the force
4. Until you get a feel for who you are defending, respect the possibility they can beat you deep and let them have the short throws underneath. Once you know you can recover on a deep throw, then start to turn up the pressure on the under throws.