In plain language
Actions must fire when the press is RELEASED, so users can slide off to cancel.
Who it affects
People with tremor or imprecise pointing who need to cancel a down-press.
How to check
Press a control, slide your finger/pointer off it, then release: the action must not fire. Worked example — A bank's mobile money-transfer screen: press down on a menu item and check whether it's already selected before you release: press-drag-release combos that select on down-press without requiring the up-event fail.
Common misconception
Making a button feel “responsive” by firing on touchstart instead of the click event actually breaks this rule, since users lose the ability to slide their finger away and cancel an accidental press. Real example: on an online clothing store's product listing page, the action fires the instant the mouse/finger goes DOWN — users can't slide off to cancel an accidental press.