Well its to do with a tail image. About a good 1cm of the end of the hackle has a scale like look that is a good tail.
Select that area of the hackle with your fingers, enough that you're happy with. Then pull the rest of the hair off about 1 inch below it to give you a good strip to tie it to the hook with. Then tie on your chenille at the shank, wrap forward, tie off.
And since theres a large bluntnose minnow population where I live, I use a sharpie and draw a line down the side to imitate their black lateral line.