An apparatus includes an instrument body and transmission assembly. The instrument body includes a motor, a switch, and an energy source (e.g., an ultrasonic transducer). The transmission assembly includes an end effector, a transmission feature (e.g., an acoustic waveguide), and a switch engagement feature. The end effector includes an active feature (e.g., a harmonic blade) in communication with the energy source. The switch engagement feature is operable to mechanically couple the energy source with the transmission feature in response to the switch engagement feature triggering the switch. A yoke of the instrument body is configured to selectively engage a translating member of the transmission assembly. A release switch is operable to selectively disengage the yoke from the translating member.