These backs are shaped to fit curved shank hooks. They are designed to give the finished a fly a jig action with the fly fishing point up.  They add a lot of weight, the largest size weighing just short of 1 gram.

They are grooved to make it easy to tie them down and they have a longitudinal slot that fits over the hook shank.

In Four sizes, see the charts on the left for a guide to weight and hook fitting.

The flies shown to left were tied by Craig McDonald, two variations of his creation Craigie's Killer . Matt Eastham has written an excellent article on the Craigie's Killer, his tying is pictured below, you can read it here

Regarding Hooks, I recommend

Dohiku 644 #10 for XL, #12 for L, #14 for M & #16 for S (Click photo to left)

Hends BL554 (Click photo to left to see how they look on these hooks)

#8 & #10 fit XL tightly, no thread under just super glue,

#12 for L or XL,

#14 for M or L and

#16 for S and M

Packs of 10