An apparatus comprises an end effector, a shaft, and a handpiece. The end effector is operable to manipulate tissue. The shaft is in communication with the end effector. The shaft includes a firing beam operable to actuate a portion of the end effector. The handpiece comprises a pivoting trigger that is operable to distally advance a firing beam driver to advance the firing beam. The handpiece also comprises an activation button and lockout assembly. The lockout assembly is movable between a locked and unlocked state. In the locked state, the lockout assembly blocks a portion of the firing beam driver from advancing. In the unlocked state, the lockout assembly allows the firing beam driver to advance. The activation button switches is operable to unlock the lockout assembly.