A lot of it was just timing and angles. I think that's what made his leg kicks so lethal, he has a knack for throwing them at the worst possible time for the other guy to try to defend. It seemed really basic at first and would probably seem pretty basic if I tried to explain here because we didn't really do anything I hadn't seen before; it was just tiny differences in timing and positioning that made huge differences in results.
As for the difference in my mindset, I used to be and should still be a good kicker, but after fracturing my tibia twice I pretty much abandoned leg kicks. Now that I feel more confident in throwing them without having them checked so hard, it's opened my game up a lot. And bringing back kicks has given more openings for my hands, which leads to more openings for kicks, so on and so forth.
I've been boxing/kickboxing for a long time, and after training with him for just two hours the guys at my home gym were like "WTF? No fair!" by the end of my first round sparring. That's about the highest compliment you can give for someone's coaching ability, IMO