Command 0x86: Motor M0 Coast
Compact protocol: 0x86
Pololu protocol: 0xAA, device ID, 0x06
Command 0x87: Motor M1 Coast
Compact protocol: 0x87
Pololu protocol: 0xAA, device ID, 0x07
These commands will set the specified motor to coast by setting the motor outputs to high impedance, which lets the motor turn freely. This is in contrast to setting the motor speed to 0, which sets the motor outputs to ground and acts as a brake, resisting motor rotation.
You can see the effect of this yourself just by using your motor. First, with your motor disconnected from anything, try rotating the output shaft and note how easily it turns. Then, short the two motor leads together and try rotating the output shaft again. You should notice significantly more resistance while the leads are shorted together. The motor-coast command is equivalent to disconnecting your motor and letting it turn freely while setting the motor speed to zero is equivalent to shorting the motor leads together.