Shower drain install

I have a new acrylic shower pan to install that came with a drain fitting that screws on from underneath the pan. That part of it I understand. There is a rubber sleeve that is about 1/8 inch thick and about 3 inches tall that looks just like the one that came out of the old shower pan. I can not figure out how that works. The rubber sleeve moves around freely in the drain fitting, but will snuggly fit around the PVC drain pipe. How can this sleeve seal the fitting to the drain pipe if it moves aroung in the fitting? I notice I can force it to fit tightly in the opening of the bottom of the fitting. The fitting has a lip or flange at the bottom that is narrow enough to squeeze on this rubber sleeve. Do I place it there and force the whole shower pan on top of the drain pipe like that? Struggling with the whole role of this rubber sleeve.
Thanks, Brian
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Brian wrote:

Set the shower pan over the pipe. (Note: set the pan in cement to provide good support.) Then slide the sleeve down between the pipe and the drain fitting. It's a tight fit and the sleeve should come pre-lubricated. Drive the sleeve down the last bit by using a blunt tool which won't injure the rubber.
More here: http://www.oatey.com/apps/catalog/showskus.asp?ctg &subctg=4&prodgrpid8#
Jim
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Thanks. You have shed bright light on this subject for me.
Brian

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