Originally from http://whirlpoolcabrio.com/whirlpool-washer-parts/
The coupler is probably the part that breaks more often than the rest. This part replaces the belt in the previous models. You will know when it is broken if your whirlpool washer does not spin or agitate.
The coupler is a vital part of the machine that connects the transmission and the motor. You can replace it with a new one by purchasing online, but be sure to check the model number.
More at coupler