Mod motors usually run with positive timing, To be perfectly honest I don't have my head 100% around this yet but, the postive timing makes the motor run faster but lowers efficiency. So when the motor is run backwards it will put more wear and tear on the motor because the timing had been set for going foward(positive timing). The trinity speed gems mod motors come with 0 degrees of timing set so they can be run in reverse and be fine. I don't know if you can change the timing on competition mod motors down to 0 degrees though. Granted you can still run a mod in reverse and have it be ok but it will put more strain on the motor.