To this day, I'm still trying to understand why my freshman calculus class (proof-based), was as effective as it was.

It was 30 students, 1 lecturer, and a whole bunch of black board panels.

I think part of it was that it felt like a conversation, as well as a game. He would lay out the pieces (assumptions, definitions), and then point us in some direction ("now how would we show X?") We'd throw out ideas if we had any, and he'd either rebut us or nudge us in the right direction. I was 100% engaged in that class - no checking of phones or surfing the net - and it was just myself and my notebook, unlike in other classes that were slides-based and I picked up the bad habit of zoning out when something familiar was being covered. You can't do that in a conversation! The material and strategies I learned in that class completely built the foundation for my math major.

I am still not sure if math is one of the few topics you can take this approach for, slides are the devil, or if the lecturer was just secretly brilliant.