Interesting question. According to several answers posted here:
http://www.practicalmachinist.com/vb/general/how-size-diode-why-cordless-drill-142630/ ,
a "flyback" diode, or pair of back-to-back diodes, is used to suppress a high voltage spike when the motor is turned on or off.
what is the purpose of a diode in a cordless drill or power screwdriver?
2 answers
This is very helpful