This is the standard horizontal stack I expect teams to play with based upon what we've seen. In this diagram, the "home" side is indicated by the cone. The other side is "away". On the field, we will use whichever side our gear is on as the "home".
In most scenarios, we will use a forehand force. In this case, that means the force is "home". Our goal is the force throws to go to the "home" side of the field and take away options to the "away" side. This is done by correctly positioning yourself on defense.
If you are guarding the person with the disc, you are the "Mark". You want to be almost parallel with the player holding the disc and facing the side of the field that was called as the force.
If you are guarding a cutter, be between them and the side of the field called out as the force.
If you are guarding the other handler, things are bit more flexible. You don't want to allow an easy swing, and you don't want to be in a position to give up a break after the swing. Because there is only one other handler in 5v5, you can cheat more to the incorrect side to block a swing pass.
How does the force work? Well by standing parallel to the sideline instead of the goal line, you can prevent throws to either side. Let's zoom in for an illustration!
Here we see the throwing lanes available to the thrower. Note how the whole left side of the field is closed off!
If the mark moves to be "straight-up", the area directly in front of the thrower is closed off, but much of the field is still open.
This matters, because the defenders on cutters, are cheating to one side in particular assuming throws will be to one side. Let's see what that looks like!
See how the #3 defender is already in the way of a potential throw? That's because they know which side the disc is going to!
If the mark is straight up, the thrower can go to either of the cutters more easily. The defenders are also more likely to be out of position.
Last, what do we mean by a break? Well, if you are the mark, and you let a throw go to the opposite side to the force, you have been broken!
Why is this bad? Well now, all of the other defenders are at a disadvantage!
It is easy to gain yardage after a break.
DON'T GET BROKEN!