This paper came out and changed the game a few years ago. It's called scratch theory and the front edge of the stone scratches tracks for the back edge to run along.

There was a sweeping summit this summer to discuss brooms and how to make them scratch the ice less because it caused players to have extremely high stats for tournaments.

Brad Gushue video showing the effect: https://www.youtube.com/watch?v=haEuz42YCdM These brooms have been made illegal in tournament play.

Some Scotties players describing the effect: https://www.youtube.com/watch?v=147vRw0kmAY

The scratch theory paper: https://www.thesalmons.org/lynn/curling/A5.pdf