[Peaker]

On a touch screen, you only have two primitive gestures from which all gestures are composed: Taps and swipes. Composition may be chronological or simultaneous (multi-touch).

This space of gestures is so narrow, that any idea within it can be explored in minutes, therefore any method that just combines these primitive gestures would be obvious.

Additionally, taps can accidentally happen (also obvious), and multi-touch is cumbersome, therefore swiping is the obvious gesture to use for unlocking.