Not being near a good computer for a little while, I'll try to invoke yur imagination.

Imagine a round hole. now imagine a smaller square hole as well but slightly off-set to the round one. The spindel sits in the round hole and the 'key' (square section rod) sits on the flat of the spindle and into the square hole. The problem is getting the tolerances right and a key the right size. But hey, if you're manufacturing stuff anyway....