Quasimode

A way to avoid ModeError when overloading input gestures. You hold down a modifier key while doing some other gesture (such as a mouse click) to modify the meaning of that gesture.

Caps Lock is a mode; Shift is a quasimode.

Rationale

Quasimodes tend not to cause ModeErrors because the quasimode is actively maintained by the user, who is keeping a muscle tensed in order to hold the key down; this muscle tension remains in the user's peripheral awareness. (In short, you can easily forget, or not notice, that the Caps Lock key is on, but you tend not to forget that you're holding the Shift key down.)

Pitfalls

Quasimodes are generally invisible, so you need to provide some form of discoverability. A brute force method is to display a status-bar message saying "ctrl-click to do X".

Quasimodes can have accessibility problems, too -- they require a certain level of finger dexterity which not everyone has.