I can see value to doing it off the trainer... but I'd suggest a wall and a mattress rather than a doorway. Or a soft grassy field once the weather permits, which is what a lot of people use. Or, if you use a doorway, wear a helmet (I'm serious).
It's not a bad idea to practice timing the unclipping and putting your foot down. You won't be able to do that on the trainer. It's really the timing, not the motion itself, that causes people to fall from failure to unclip (FTU).
So if you're not actually moving, you do need something to hold you up, but it's a doorway, then you've got the solid other side of the doorway right in the path of your potential fall. Not good. Plus (which would be true for a wall too), you'd be getting in the habit of leaning to the opposite side of the foot you're unclipping, which isn't really a great idea either. So grass, if you can.
Now that I think about it - a couple of months ago, a few of the experienced riders in my club were discussing FTUs and one of them pointed out that it's really all about countersteering, and I'll be danged, I think he's right. When you're ready to get off, turn the handlebars away from the foot you're going to unclip. That'll induce the bike to lean toward that foot, and you'll be properly positioned for unclipping and putting the foot down.
Last edited by OakLeaf; 02-19-2010 at 03:32 AM.
Speed comes from what you put behind you. - Judi Ketteler