Dubbing Spinner Set of 3 Heads, with Hair Packer

I acquired the 3pc Anglerhaus dubbing spinner from a friend. The length of the handle can be cumbersome at times. The slit in the endorsement the aluminum handle is used for hair stacking by sliding it onto the shank of the hook, convenient.

Works great, love th options of the dubbing tool. I'm just not understanding the slit down the side of the aluminum shaft.