I have used them in Wing Tsun much like Zepedawingchun described, and even more in Eskrima. For this, I loop the band around a post and grip the band just below the handles. Facing away from the post, I do reps of our Cinco Tiros, or five basic strikes. To increase resistance, you just move further from the post. It has helped me with speed, "snap" and power.
Lately, I've gotten away from the bands and have been doing more work with dumbells, but I really think I was getting more functional results with the bands. Maybe it's time to change my routine again.