Hammerfist  is  used in ground-and-pound  in MMA to  avoid  damaging the bones  of the hand . The arm is moved like a swinging hammer, or it’s moved horizontally  like a  backfist.  The power of impact is the lower  pad  of the palm  below the little finger.

Hammerfist power is delivered by transferring body weight through shoulder , down the bent arm and into the fist.

Check this video on of the forms of  throwing the hammerfist: http://www.youtube.com/watch?v=DA6HNk8gxnY