Definition
The force is a defensive choice about which side of the field the offence is allowed to throw to. The marker stands so that one side of the thrower is closed off by their body and arms; the other side is left open. The team agrees the force before the point — typically forehand or backhand — and every defender's positioning downfield is built off it.
In Context
The force is the most fundamental defensive tool in the game. It tells the offence which throws are easy and which are hard, and it tells the defence where to stand. A forehand force means: we are willing to give up forehand throws, and we are taking away backhand. Every downfield defender then shades to the forehand side because that is where the disc is going.
A force is only useful when it is effective — see Effective Force — meaning the marker is settled, balanced, and able to slide with the thrower's pivots.
Our offensive principle Move the Disc is partly designed to attack the force before it sets. A thrower who releases inside the 2 Second Window is throwing while the mark is still arriving — meaning the force has not yet effectively chosen a side, and both throws are available.