Rebound in shocks controls how fast or slow the stored energy from the compressed spring during compression is released. Too fast, as in my case, allows the spring to release too quickly giving you that basket ball feeling of being too bouncy. Too slow, and the spring shock and tire release at a much slower rate. Finding the perfect balance of compression and rebound is key for any enthusiast or racer for optimal handling and performance.
With the exact springs front and rear Im running, Im good with the compression valving, but due to the heavier springs I need to add more rebound valving to slow down the release of the spring.
One very important thing I forgot to mention earlier was this. I had the Bilsteins for about 6months on the stock front and rear springs before I added the aftermarket springs. And, according to Bilstein, they valved the shocks for the stock spring rates. The Bilsteins worked better when I had the stock springs.
Hope this helps.