I use to train with Keith Anderson who was one of the best descenders around.
He tried to teach me on many occasions how to descend at speed.( running behind me pushing me and tied to a cord as he shot off pulling me
)
The " art" is leaning forward, relaxing and very lightly touching the ground with the ball of the foot/toes.
Once on Loughrigg in Ambleside and I said right show me how its done.
We stood at the top of a very steep descent, Keith said just follow me.
We set off together him bounding away like a gazelle me trying my hardest to relax, breathe normally and follow.
All of a sudden he leaped off a (8/10 foot) cliff, i came to an abrupt halt nearly messing myself.
Keith had landed,rolled over & back on his feet as if nothing had happened then carried on to the bottom, turning and laughing at me.
For me a lesson learned...I could run down hill but I was never going to be in the same league as a truly great descender