Shake to undo

Shake the phone and iOS raises an alert offering to undo the last edit, with Cancel beside it. Shake again after that and the alert offers the redo. It has been a system gesture since the first iPhone, handled by the operating system rather than by the app: the accelerometer reports a motion past a threshold, the frontmost responder is asked whether it has anything to reverse, and the alert appears if it does. Nothing on screen changes until the reader confirms, which is the one piece of the design that is unambiguously right, because a gesture this easy to make by accident cannot be allowed to act on its own.

It is worth separating from shake, which is the same word for the opposite direction of travel. A shake animation is a refusal the interface makes: a password field wobbles to say no. Shake to undo is a gesture the reader makes, with the whole device, and the interface answers it. One is output and one is input, and the only thing they share is the shape of the movement, so a sentence that says “the app shakes” and a sentence that says “you shake the phone” are describing unrelated mechanics.

The critique is discoverability. There is no affordance: nothing on screen suggests the gesture exists, nothing hints at it the first time an edit is made, and a reader who never happens to read about it will never find it. That is the defining problem of a motion gesture, and it is why undo still needs a control somewhere reachable, in a menu, on a toolbar, or behind a keyboard shortcut, with the shake as a shortcut for people who already know. The false positives are the other half: a phone in a hand on a bus, or a person walking briskly, produces motion that reads as a shake, and the alert interrupting an unrelated task is how most people first learn the feature exists.

Motion as an input has an accessibility cost that WCAG names directly. Success criterion 2.5.4 (Motion Actuation) requires that anything operable by moving the device also be operable through conventional controls, and that the motion trigger can be switched off, precisely because a tremor, a wheelchair over a kerb, or a phone mounted on a chair produces motion nobody meant. iOS provides the switch under accessibility settings, and a good many people turn it off the first time an alert appears out of nowhere. Build the gesture as a convenience layered on a control, never as the only route to the action.