I have a set somewhere but never used them. I know they are used to shield the collar / hackle fibers from the tying off operation at the head of a fly. It's common to get the barbs pulled into the head making a mess and affecting the head cement part etc.
To be honest I use either my wide gap whip finisher and wet fingers to apply a little "mousse" and/or whip finish with the fingers holding a bodkin which helps but frankly using a guard would be much more effective at times.
I also took the remedial verses preventative route by purchasing an expensive pair of capilary snips which can remove individual barbules from under thread. Now that I think of it... I'd save time with a guard.
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