Gotta love the internet, so here's my $0.02 worth:
You're right, shock rebound has nothing to do with adding to the spring rate.
What it does is slightly reduce the damping on the rebound stroke.
If you have no rebound at all, the damping will be relatively equal between the compression and the rebound stroke. When you push the shaft in and back out at the same speed, it should feel the same.
But when you add rebound, the effective damping during compression will be slightly higher, and the rebound damping will be slightly lower. Pushing the shaft in should feel harder to do at the same speed (especially during the top half of the stroke) than pulling it back out.
With this configuration the spring will return the wheel back down quicker.
I also experimented with the 2-stage pistons at one point. But they produce an extreme difference between compression/rebound damping, which I found to be overkill.