This is a great exercise to improve the size of your biceps, yet it's rarely used, mostly because it doesn't allow for a lot of weight to be lifted and some egos just can't deal with that. Also, this exercise requires perfect form and control – things the average gym-bro doesn't have.
The drag curl emphasizes the brachialis and outer portion of the biceps (the long head). The number one rule is this: squeeze extra hard at the top. Here's how to do it:
For this exercise to work you should also use a controlled tempo. You'll need to use less weight than you typically use with a standard curl, so leave the ego in the locker room.