Yup. little wings forged into the side of the shank, yet the rest of the shank is still round.
The end is squared off blunt.
So you press it into a tight hole drilled into the handle and the little wings fight their way in until the shank bottoms out on the blunt end.
The wood recovers some after the wings pass, and this plus the wings themselves keep the shaft from spinning.
I can't believe you never pulled one apart. :)
Get a total junker and clamp in a vise just below the ferrule. Take a prybar or very large screwdriver and slip it in under the ferrule. Pry it up. It'll move 1/4 to 1/2 inch. Take another bite on the shank near the ferule, and do it again.
Pretty soon you will work it off, and you can see for yourself.
There might be times you want to swap one handle onto another shank. This is how.
yours Scott