Does a modified "continuous rotation" servo typically create an H-bridge motor controller?
The servo has an H bridge motor driver along with a PID controller of some sort that attempts to run the motor until the potentiometer position matches the PWM signal from the controller.  Converting the servo to 'continuous rotation' involves simply breaking the feedback loop by disconnecting the potentiometer from the motor shaft.  When a PWM signal is applied, the controller inside the servo attempts to turn the servo until the potentiometer is moved to the correct position.  However, since it is no longer connected to the shaft, it does not move and the motor just keeps spinning.