I've never been able to understand why the rear technically "rises" on acceleration either.
If you watch this clip of a bike on a dyno you can see it happening.
http://www.youtube.com/watch?v=ymcOLPaV9Ls
In practice of course it doesn't move that much because of the counter effort from weight transfer, but the principal that the force acts with the spring to "stiffen" the rear can be felt.