By itself, it may not stop a winch from unwinding itself, but if you use a Victor set in Brake mode along with a motor with a worm gear (Van Door motor, perhaps?) then it would be pretty hard to unspool itself (although not impossible).
We've done some testing with the brake mode on drive motors, and it makes the robot stop much faster than in coast, so it definitely has an effect, if the motor is spinning at high speeds. But anyone who knows the fun hand rules from physics can figure that out
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