Distributor lock down collar?

I know what he is talking about. It's a collar that attaches to the bottom of the distributor shaft. It is "somehow" supposed to stop the distributor shaft from bouncing and changing the timing.

The distributor already has a collar under the housing with a roll pin in it. That collar limits how far the shaft can be pushed "up". Like I said, I don't know how the aftermarket collars that go on the bottom of the shaft work. I'll see if I can find a picture.