A front hand strike with whip like attributes. Back fist is a fast, surprising usually done to surprise the opponent right after a flurry. (attack, attack, attack, pause relax shoulders, backfist)
The key is to treat you hand like a whip and really let it stap. When you start out practising back fist you may want to limit your reps to 10 as it puts a lot of strain on your elbow joint.
If you have any questions regarding this technique you can post them in the comments below.
