The RBS knot (pronounced "arbs") is named after Richard B. Schroeder, its inventor. It was quite popular before "everyone" switched to mechanical ascenders. The knot is tied by simply clipping the carabiner to the sling, spiraling the carabiner around the standing line twice, clipping the sling again, and locking the carabiner. This can be done in only a few seconds. The relative orientation of the carabiner and the standing line can take on a number of forms, but they all seem to work. The RBS knot is easily moved, but it tends to fall if unloaded. As with all carabiner knots, a locking carabiner is recommended.
The inverted RBS knot also works, but it never became popular, probably more because it was perceived to be "incorrect" than for any real disadvantage.
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©2007, Gary D. Storrick