Why it's on the list: In contrast to traditional barbell curls where you keep your elbows pinned at your sides, you'll actually push them backward, keeping the bar close to your torso as you bring it up. This reduces the range of motion, so don't expect to take it up very high.
Because the bar moves vertically up and down, it can also be done effectively on a Smith machine. This is a favorite biceps-building "hack" of Kris Gethin in his popular 8-Week Hardcore Daily Video Trainer.
Drag Curl Variations for Biceps Growth:
In your workout: You can program this just like any other barbell curl, heavy in the beginning or lighter in the middle to end of your workout. Want to make it extra tough? Extend the negative portion of the rep to 3-5 seconds per rep. Repeat for 5-8 reps, perhaps having a spotter help you with the concentric (lifting) portion as needed.