The motor coupling goes between the motor and the transmission. If it fails the washer will not spin or agitate. If the washer fills with water but then does not agitate, but you hear the motor running, the coupler could be the problem. You may hear some clicking noises as the broken pieces spin around. You may also see a pile of black rubber pieces on the floor under the middle of the machine. You can follow the steps in this video to help you do the repair.