RSL vs Skyhook
For those who don’t know, the RSL (Reserve Static Line) and Skyhook serve similar purposes, but are not identical!
Getting out your reserve parachute safely and quickly can mean the difference between life, and well that unmentionable. :)
If you cut away your main parachute, your RSL pulls out the reserve pin, which initiates the opening of your reserve canopy. Think of your main as a giant pilot parachute at this point. The drag of the main parachute is more than sufficient to pull the reserve pin out.
This can be considered a good thing in most circumstances. If you cut away your main parachute, but don’t react quickly enough to deploy your reserve, you can rely on your RSL to assist you here.
The RSL has a quick disconnect, which can be disconnected. This is important in some situations. Lets say you land in a tree. Before cutting away your main (in a safe manner when getting out or down) you should disconnect your RSL. Otherwise…you may feel even more awkward when explaining that after landing in the tree, you deployed your reserve canopy. (A few more cases of beer?)
The Skyhook is much more aggressive than the RSL. It pulls out the reserve parachute using the main deployment parachute as your large pilot parachute. A much better explanation is offered here on Wikipedia.
Edited for clarification: The key here is the RSL pulls ONLY the reserve pin to initiate the reserve deployment. The Skyhook takes it a step further by also pulling the reserve out of its container.