I guess I've been doing this sort of naturally in my workouts for a long time. It started when my buddy and I used to do burnout curls. We'd go dumbell curls from 50 all the way down to exhaustion.
More recently, I've just been getting on a machine and starting at 140 or so, getting to the top of my curl motion and then letting the weight drag me down as slowly as possible. Drop to the next weight and repeat all the way down to 20. It looks silly when you're really straining to keep 20 pounds from pulling your arms down on a curl, but it sure as hell works. I try to close with one of those sets for every workout I do, be it curls, tricep extensions, etc.