Powerful pneumatic grinders have a governor that regulates the air inlet as a function of the speed. When speed increases, the inlet gets smaller to limit it to the maximum speed defined for the tool. This mechanism is developed to work at a certain air pressure; if the air pressure increases significantly, the speed will become too high. This is where the overspeed protection comes in. See it in action:

At the correct pressure, air travels from the air inlet through the motor to the exhaust, without any intervention from the shut-off mechanism. The tool can work at its full effect.

If pressure increases heavily and the wheel speed significantly exceeds the maximum speed defined for the tool, the centrifugal force will liberate a steel ball that will block the air inlet. This shuts off the air flow and the tool stops. This happens instantly, in a fraction of a second, before the grinding wheel has the time to reach a speed where it could burst and send fragments into the path of the operator or someone else.

IMPORTANT: To restart the tool, you need to carry out maintenance; the steel ball has to be removed from the air flow and put back in its initial position. An actioned shut-off typically signals an abnormality with the air flow, so we highly recommend you perform airline diagnostics and investigate the root cause.