You have to relax!I've never been so scared and it takes a lot!! It was the fact my head was saying slow down as I was going down hill and my legs were saying I'm trying to lol
However, will admit that I switched to a SS freewheel for my training/leisure bike. I found on a low fixed gear, it is the descents that are the most demanding and puts a strain on the arms and shoulders as well. With a freewheel you can go as fast as gravity allows and you have time to recover on a descent. On the same loops. My SS is often faster than my geared bike.
I have another bike with a 50x14 or 15 fixed that I use for timetrials. The bigger gear doesn't run out of revs and is fine on the descents.