HF's and Piriforis muscles are engaged primarily when you pull up and over the top of the pedal stroke. The picture posted by Hermes is a good example of HF and Piriformis engagement when pulling the trail leg through to become the lead leg. The HF and Piriformis need to be stretched out pre and post exercise and need to be strengthened, generally because so little focus is placed on them, especially Piriformis. Tight HF's and Piriformis will also contribute to low back problems, soreness and premature fatigue, especially while on your bike.
Here are a couple of sites that should help you.
I suggest you engage a very competent personal trainer to help you understand how to stretch and work these muscles. Incorrect Piriformis stretching can put a lot of torque on your knee.
http://www.google.com/imgres?imgurl=...1t:429,r:0,s:0

http://sportsmedicine.about.com/od/f...formis_str.htm

http://www.sports-injury-info.com/hi...or-injury.html

Good luck