Ok, Moggo - your first part makes sense, but as I understand it was originally introduced as a security device to avoid the downwards rebound bang you are describing. Now, the actual length of the rebound spring creates a force counteracting the main spring, a preload. At least I cannot see any reason for this preload, and there are many advocating cutting the rebound spring so it just kisses the underside of the hub/axle. The length itself is not important, and I am of course aware of it being shorter than the main spring. The preload and the oscillating (nodding dog) movement of the axle is the main issue.
