Momentary or constant? if momentary I assume there is a relay or
capacitor in the trigger that applies the voltage.
Almost all of these work with the motor turned into a generator when the
trigger switch is 'off' (effectively shorting the motor). The most
common fault, by far, is the trigger switch has succumbed to dust
intrusion (easily fixed with a blast of compressed air). Sometimes the
brushes get worn to the point that they no longer seat properly and
cause the same braking problems.
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