Single coverage in the "red zone" – the area between the twenty-yard line and the goal line – is usually designed to prevent receivers from slanting towards the middle of the field. These types of routes are difficult to stop in the red zone because this area is usually congested with bodies colliding, crossing, and weaving in different directions. Although illegal, defenders are easily picked or screened by opposing receivers and sometimes by their own teammates; this is illegal yet difficult to enforce in short field, congested situations. To avoid this, it is often favorable for cornerbacks to either: "switch" assignments, where he will agree beforehand to trade assignments with one of his fellow defenders in the event that the receivers criss-cross as the play begins, or alternatively, a corner may instead line up very close to the receiver at the line of scrimmage to force or "jam" him toward the sideline (outside) without violating the 5-yard no-touch rule. Corners often refer to this second style of coverage as the "man under" technique.