The SilentStepStick Protector is an addon module for StepStick and Pololu A4988 compatible stepper motor drivers. The board provides flyback diodes (freewheeling diodes) for the motor outputs, so that they are also protected against induction voltages in the unpowered state of the driver.
The v2 Protector adds an additional diode from the logic supply voltage VIO (anode) to the motor supply voltage VM (cathode), to ensure the correct powering of SilentStepStick drivers. When the motor supply voltage VM is not present then it is not allowed to enable the driver (motor outputs) and there should be no loads (e.g. fans) on VM, because the current will be drawn from the logic supply VIO.