@aragon17 Then why are they called high speed motors? Also, how might the PTC fuse affect the turbo motors? Sorry for my ignorance.
@antichamber is saying that in comparison to the turbo motors. Hi-Speed gears are still faster than the standard torque gears, but have a lower torque than the latter. There's an inverse relationship between torque and speed for the same max power capacity, and changing gears affect the balance between torque and speed.
Standard > High Speed > Turbo
Turbo > High Speed > Standard
The PTC fuse is designed to protect the motors from damaging themselves when attempting to work against an excessive load. It's a thermoresistor whose resistance varies with temperature. What happens is when the motors are subject to an excessive load (such as trying to transport a heavy weight or working against a force too great for the motor to overcome), the current inside the motor heats up the fuse faster than the fuse can cool and as a result, the PTC generates more and more electrical resistance until it reaches a tipping point in terms of temperature, at which point it just sends out a lot of resistance that the current is essentially blocked and the motor won't run. It's this yellow thing on top of the motor (the actual motor inside the motor casing) in the image below:
Once the fuse trips, the only way for the motor to return to normal is to let the PTC fuse cool down over time. In case you get the brilliant idea to just take the fuse out, let me remind you that that is extremely illegal in the VEX Robotics Competition and if referees at a tournament discover your team doing this, your team will be DQed from the whole tournament, no exceptions. There have been teams at VEX Worlds that got disqualified from the entire event at the inspection table over this.
If you want to keep the PTC fuse happy, you must not subject the motor to a load that it can't bear. If you want the motor to carry a heavier load, you "downgrade" the motor to Standard or High Speed. The motor will be slower though. Other ways to keep the motors from overheating include adding more motors to the same mechanism (to distribute the load among multiple motors), reducing the weight of the robot or mechanism itself, etc.