Originally Posted by fyrstormer

The main purpose of submerging a motor during a wet break-in is to subject the motor to significant drag, so the motor will have to work extremely hard for an extended period of time. The significant drag limits the motor's speed, which reduces the magnetic flux inside the can. The reduced magnetic flux is unable to generate as much inductive resistance in the armature coils as it normally would if the motor were spinning freely. The lower resistance allows the battery to push more amperage through the motor, and because that amperage is being pushed through a very tiny contact area between the brand-new brushes and the commutator, the contacting surfaces of the brushes get extremely hot and start to disintegrate. This accelerates the break-in process by an order of magnitude -- but it can also destroy soft brushes in a matter of seconds, as I and many other people have discovered over the years. A wet break-in is only useful (or safe) for motors with extremely hard brushes that would take an extremely long time to break-in without the continuous drag of running underwater. For brushed motors with softer brushes, a few full-throttle accelerations will put enough load on the motor to break-in the brushes, and then the commutator can be cleaned with electronics cleaner spray.

Regarding brush debris: Submerging the motor in water allows brush debris to be washed away instantly by the churning water inside the motor, whereas applying a few drops of water directly to the brushes just causes the debris to form a grinding paste that stays on the brushes. That's why the brushes broke-in so quickly for you. If it works, it works, but there's a significant risk of the brushes being damaged by so much abrasive material in such a small area.

(source for magnetic flux explanation: John Rob Holmes of Holmes Hobbies LLC)