I would argue against taking long "bounding" strides downhill, mainly because of the injury risk. To go super-fast downhill you should try to run with the hill, let it take you down, trust in your ability, and just let go. But DON'T overstride. Keep leg tempo fast, with quick turnover; all you need to do downhill is change the support leg on which your body is riding, there is no need to "push off" or get forward momentum like when running up or on the flat, so just concentrate on lifting your foot from the ground quickly, while the other falls to the ground with gravity, then pull that one...and so on. Gravity will do all the work. But maintain focus and concentration at all times because going fast downhill is massively risky.
Only use hamstring muscles to do this. Just to lift the support foot from the ground, one at a time. Imagine you are running on hot coals.
Quad muscles should not be excessively used - if you get tired quads going downhill it's because your strides are too long and each time a foot hits the ground the leg must brake the fall big-time. If you use quicker tempo, each time the leg hits the floor the breaking force required is much smaller and muscle elasticity goes a lot of the way to doing this for you, without the need to overwork the quad. You need your quads to be fully rested for the ascents of these hills! So overworking them on the descents is bad news.
It is always tempting to reach out downhill and just go for it, and you do go faster, but it's not maintainable speed because you will either get injured (by turning and ankle or worse) or tired. Developing the ability to employ rapid turnover to avoid overstriding is difficult though, because you're effectively "sprinting" - having to fire different muscles quickly - and un-trained legs end up working against themselves as the muscles can't quite fire in the right order. So when you start to employ this technique, you may well go slower and feel more tired. This is probably why most of us overstride downhill - because it's the "easiest" thing to do. But if you keep practising and practising, your legs will develop the ability to move quickly like that without working against themselves and getting tired. Persevere, because once you develop the ability to run downhill like Ian Holmes you will be unstoppable.
Hope this helps. Take a look at these downhill running clips: clip 1 and clip 2
November 2007