In reply to Anonymous:
They used to be used on etriers with a long cord, so you could retreive them. Modern techniques don't use this system.
Nowadays, you clove-hitch a fifi to your belay loop, so it's about 6" to 9" long according to taste.
When you stand in your top step, or the highest one you want, you hook the fifi into the krab attached to your gear.
Then you can sit and rest, or continue stretching to place your next piece. Then you're tensioning on the hook for stability, rather than holding on with one hand and flaming your arms.
In the old days, you would clip your rope and your second would hold you steady on tension, but poor communication would cause all sorts of problems, especially when you needed to stretch up.
It is possible to reach much higher than normal by tensioning up, (having your hook going
down to your gear), - that takes practice and nerve, but saves time with less placements.
The hook is better than a krab, as it's so fast to hook in and fast to release when you are needing to move it from one piece to the next in less time than you've got and the one arm you're hanging on is screaming for a rest.
You
can hook up your daisy loops, but with practice you will find you can move it from piece to piece without wasting time in between.