What's this called where they put a kind of dimple in the seat tube for wheel to tuck into?
In general, it's called a dimpled seat tube. In that case, as WP notes, it's called misdirected energy since the tyre is going to hit the bridge before it gets to the seat tube, dimpled or not.
In general, it's called a dimpled seat tube. In that case, as WP notes, it's called misdirected energy since the tyre is going to hit the bridge before it gets to the seat tube, dimpled or not.