These add pullstart capability to your YYBC workhorse yoyo. Depending on printer you may have to scale up or down 1% or so to fit, but the current size printed on my MK4S fits both of my workhorses perfectly.
Notches in the side are for easy removal since it wasn't designed as a capped yoyo, stick in a key or something and pop the cap out when not needed.
The author marked this model as their own original creation.