Burnham
Woods walker
I'm just wondering how you use it!
Here's some pictures. The first shows standard single line rigging for rappel. Second shows rappel with additional friction for heavier loading, third shows lock-off. You would rig it similarly if you wanted to use it for controlled lowering.
This device allows variable friction on the fly w/o re-rigging, and locks/unlocks without any slippage.
The lock-off method that Nick linked for a deaf 8 is a poor option, imo...you have to place the rope in the position of least friction and somehow manage to hold your position on rope while you tie the mule knot...not a practical solution in my experience.
The last pic is my standard rappel system from a spar, on doubled rope and retreivable.