In reply to AlanLittle:
> Surely at the first bolt friction is a good thing if it helps to keep your belayer on the ground, and impact force is not the major consideration because it's a bolt? Or am I missing something?
I like to use them on first bolt to reduce friction as long as long as I judge it safe to do so. Depends on the route, the difficulty of that section of climbing, the weight of my belayer and position of the bolts. But the first bolt is often the one where most friction is encountered because the belayer may stand back a bit.
A few weeks ago I did a route where the first crux was 1st to 2nd bolt. I fell off just before the second bolt, my belayer got yanked up and I was close to the ground. I considered changing the biner to a normal one to shorten the fall. But decided not to. I had taken the max fall and didn't hit the ground and decided I preferred the longer softer fall to a shorter harsher one. When there's less rope out the falls tend to be even harder because there's less rope to stretch and soften the fall.
But generally its definitely something to be very aware of when using these biner's, particularly if your belayer is lighter than you are.