A rope block is a method of preventing the rope from sliding through the anchor rigging in one direction but allowing it to slide through in the other direction.
The releasable figure 8 block functions as a rope block and as a contingency anchor. It functions as a contingency anchor because you can undo the last twist around the figure 8 and lower someone if they get stuck while rappelling. It functions as a block because it lets you rappel single strand and because you don't have to remove the releasable figure 8 block before the last person rappels down.
To rig the releasable figure 8 block:
The releasable figure 8 block functions as a rope block and as a contingency anchor.Rope blocks allow you to rappel single strand, set the length of your rope for swimming disconnects, or rappel down cliffs that are longer than half your rope.
This method requires an extra figure 8 descender and a quickdraw.
Depending on your rope size, you may have to fiddle around with twists so you get the right amount of friction.