Coupling clutch is a part of the mechanism that is used to transmit rotational motion torque from the engine shaft to the shaft of the mating nozzle.
Blenders usually use gear couplings - they compensate for shaft displacements using the provided side clearances of the mating teeth.
It is also a safety clutch - if overloaded, it fails in the first place to prevent engine damage. For this reason, it is made of plastic - replacing a plastic coupling will cost less than replacing the engine.