An apparatus for driving an end effector coupled to an ultrasonic drive system of a surgical instrument, the apparatus comprising: a generator configured to couple to an ultrasonic drive system of an ultrasonic instrument, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide, wherein the ultrasonic drive system is configured to resonate at a resonant frequency, wherein the generator is configured to: generate a first ultrasonic drive signal at a first frequency different from the resonant frequency; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period to provide a first level of mechanical vibration to the end effector suitable for separating a first tissue from a second tissue; automatically generate, after the first period, a second ultrasonic drive signal at the resonant frequency; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period to provide a second level of mechanical vibration to the end effector suitable for transecting and sealing the first tissue; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods. a generator configured to couple to an ultrasonic drive system of an ultrasonic instrument, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide, wherein the ultrasonic drive system is configured to resonate at a resonant frequency, wherein the generator is configured to: generate a first ultrasonic drive signal at a first frequency different from the resonant frequency; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period to provide a first level of mechanical vibration to the end effector suitable for separating a first tissue from a second tissue; automatically generate, after the first period, a second ultrasonic drive signal at the resonant frequency; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period to provide a second level of mechanical vibration to the end effector suitable for transecting and sealing the first tissue; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods. generate a first ultrasonic drive signal at a first frequency different from the resonant frequency; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period to provide a first level of mechanical vibration to the end effector suitable for separating a first tissue from a second tissue; automatically generate, after the first period, a second ultrasonic drive signal at the resonant frequency; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period to provide a second level of mechanical vibration to the end effector suitable for transecting and sealing the first tissue; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods.