This video will show you the more safe and more efficiency way to break in motor by "Using a Slave motor to break in "
When properly broken in, a motor can achieve a 10% to 20% increase in power over a motor that has not been broken in.
Using a slave motor allows the motor to break in without running under its own power (which heats everything up and glazes the bushings, preventing a quick break in). Properly seating the bushings will increase performance by reducing drag. If you run the motor under its own power to break it in, you risk heating and glazing the bushing surface where the armature pole makes contact. A glazed surface will be harder to wear in. When running the motor under its own power arching occurs that may pit the brushes and communicator before the communicator can be properly worn in by the brushes. Using a slave motor will drastically reduce the amount of arching. Note that the only way to remove arching completely on the new motor would be to remove one brush and break each brush in individually.