Some horizontal surfaces provide a supported firing position, allowing you to steady your weapon for massive bonuses to weapon control. You can anchor to objects using the [Z] key, but keep in mind that not all objects can be anchored to. Those anchor-able object will be marked with a dialog box when you approach them. Movement changes slightly when supported – lateral movement ([A] and [D] keys by default) will strafe along the surface. Backward movement ([S] by default) and turning too far with the mouse will remove you from the supported position.