The Drag Effect

The skip of my curling team competed in the Brier a few years ago. I make him look pretty talented. Recently he was talking about the drag effect. The only thing I understood about it during our match is that when two stones are touching they do not behave exactly like billiard balls.
Take a look at this shot.



That is not how billiard balls would have behaved in that situation. Basically, the struck rock and the yellow rock behind it were frozen together on their striking surface. The struck rock then transfered some of its momentum to the yellow rock. A more detailed explanation of the drag effect is available here.