If you run much on trails, you'll recognize pretty quickly if you're the sort of person who likes to bomb down hills at top speed, or the type of person who takes it nice and slow. I think that very often it's one's level of comfort with running down hills that makes the difference- it can either be quite scary or quite exhilarating to be right on the edge of control.
The ability to run downhill fast is pretty important if you do much racing on trails- you can make up a lot of time if you can really open up your speed on the downhills, and if you are able to minimize the braking, you can save your legs as well.
Running downhill fast has two basic components- gravity (which everyone has going for them) and confidence, which can be gained through practice. I've found that running down hills on snow covered trails is great practice for learning to run downhill fast, for two main reasons- when trails are snow covered, they're slippery. That may not seem like a benefit, but it is, and I'll come to that in a moment. The other great thing about snow covered trails is that it hurts less to fall in snow than to fall on trails not covered in snow, so you can throw caution to the wind a bit more.
Before I get to the snow bit of my thoughts, I should mention the most important thing about going downhill fast- your stride. A lot of people tend to lengthen their stride when they go down hills- they take long steps, and reach their leg out in front of them. That does two things- it speeds them up as they're in the air (which isn't necessarily a bad thing), but then causes them to brake when they land with their leg out in front of their centre of gravity, which is very hard on knees and quads. I've found that fast turnover is the key. I try to keep my stride short and fast, turning over my legs as quickly as is comfortable. The more important thing though is to focus on landing on your forefoot- if you land on the forefoot and slide, then your heel can catch you, but if you land on your heel and slide, your butt is the next thing that is going to touch the ground.
This is where the snow comes in. When you run downhill in snow, you're going to slide on pretty much every step. If you can get used to that slide- used to adjusting your balance and carrying on, then that skill is going to carry over to when you slide on the dirt when the trails aren't snow covered, which will then translate into confidence for going faster down hills.
Winter is a great time to run on trails- the skills your can practice in the winter will definitely be of benefit when the spring comes and the trails are clear again.
If you take the time to practice going fast now, you'll definitley reap the rewardsin