Blue 1 has the disc. At the moment, Blue 1 can throw anywhere she wants. This is bad for red.
Red wants to force 'flick' or 'forehand'. This means they only want Blue 1 to throw to one side of the field.
By marking on this side of Blue 1, Red 1 prevents half of Blue 1's throws.
Blue 1 is now being forced to throw to her 'forehand' side by Red 1.
Red 2 and 3 know the Blue 1 must throw to that side, so begin to guard on this side of the field.
By staying on that side, they make it hard for Blue 2 and 3 to receive the disc.
Working together, Red team forces Blue to throw into one part of the field, and then guards that side of the field.
If Red 3 thinks Blue 3 is likely to score from a long pass, she can move herself closer to the endzone, to make that harder.
Red 3 is guarding the 'deep' area to stop this kind of throw.
Let's reset.
Now, Red 2 thinks Blue 2 is likely to want a short throw close to Blue 1. He moves closer to the disc to make that harder.
Red 2 guards like this to stop this kind of throw.
It is likely that Blue 2 will be forced to change direction. Red 2 must then work hard to stay close to Blue 2.
As a defender, you must always be thinking about the force, and where the other team want to pass to. Be active and guard these danger areas. Working together and being smart is the key to this kind of defense.