Some founders and key executives negotiate into their equity arrangements that they will be entitled to some form of acceleration of the vesting of their equity upon the occurrence of a triggering event. Often, the triggering event is the sale of the company, but can also be an involuntary termination of employment.
Acceleration triggered solely by the sale of the company is called “single-trigger” acceleration and results in some or all of the vesting restriction lapsing in connection with the sale. This is designed to reward the employee for his/her contribution to a “successful” outcome for the company. Acceleration upon an involuntary termination of employment without cause would also be called “single trigger” acceleration. “Double-trigger” acceleration, which is more common, would occur upon an involuntary termination without cause following a sale of the company.