When I wrote the first and second part of this series I didn’t own a bottom whorl spindle so I couldn’t really comment on it’s benefits and design. I’ve since acquired one from the same seller I purchased my top whorl from (Fiber Artists Supply Co.)
Soon after purchasing it I noticed the difference in the design from my top-whorl. For one: The whorl (or weight) of the spindle is at the bottom (hence bottom whorl spindle). To offset their being a lack of a notch there is a hook on the top of the spindle for you to suspend your recently spun single as you spin more fiber.
It makes a great plying spindle because of how fast it spins and how heavy the whorl feels. The upside to this is because the weight is at the bottom it’s not likely you’ll lose singles as you spin and the more you build up the cop (spun singles) the better it spins.
I made a lace weight single 2-ply yarn using this to test it out when I first purchased it and while the yarn was OK I notice lace weight isn’t really the ideal yarn for this type of spindle. It’s suited to make heavier weighted yarns (sport, dk, etc.)
It came out more dense and less soft to the touch than the yarns I plied on my support spindles. (That’s a discussion for another post.)
*This is my personal opinion and experiences I’ve encountered while learning how to spin and trying different tools to achieve my goals.