An equity provision that fully vests unvested shares the moment a single defined event occurs, almost always the sale or acquisition of the company, regardless of whether the employee stays through the transition.
Snap's S-1 disclosed in early 2017 that Evan Spiegel and Bobby Murphy held employment agreements with single-trigger acceleration on a change of control, while the broader employee base was on standard double-trigger terms. The disclosure became a case study in how acceleration tiering quietly reinforces founder control without showing up on the cap table.