It is a fundamental skill for defenders to know and understand where to block off the area of the field a handler can throw. In these cases, you're forcing the handler to throw to a certain side of the field. Mostly designated as Home or Away.
The Home side is the side of the field where everyone else on the team is standing/sitting. It can also be thought of as the side of the field where everyone puts their bags down.
The Away side is the side of the field opposite of where everyone else is or opposite of where the bags are.
Some people say force flick or force away, or even force left or force right but sometimes it gets confusing as players keep on switching sides of the field and orientation is different on opposite ends.
In the initial setup, defender 6 's body is towards the side of the field where everyone is. His back is turned to that side, the Home side. As a result, 6 is forcing away.
In this setup, 6's back is facing the side of the field that has no one on the sideline. 6 is forcing H2 to throw to the side of the field where the people on the sidelines are --6 is forcing Home.
When defenders initially setup, on the line, they'll call out a side to force on, either "Force Away" or"Force Home". As noticed in the previous steps, when 6 is forcing Away, defenders (red) 1 to 4 move to the Away side blocking the paths for cutters (blue) 1 to 4 who want to run towards that side of the field.
When 6 is forcing to the Home side, defenders 1 to 4 move to the other side of cutters blocking their paths to the home side.