This item was new for Ultimate, and actually from the Super Mario Galaxy games. I didn’t realize that until I looked it up. Now it’s pretty obvious.
It’s technically a throwing item but you throw it to lay it as a trap.
Whoever falls through it gets launched. Even if they don’t hit the edge of the screen, they’ll get hurt. I always like when my enemies fall into mine.
But you can land inside your own star, so be careful! It’s not a perfect item.