This is pretty advanced stuff, so if you’ve never heard of it, don’t beat yourself up. And it’s pretty Instructor specific, so, even if you are a pretty advanced diver, you still may not have heard of Rich Right Lean Left. The concept comes because if you’re diving with multiple different gas mixes, you can often be loaded up with lots of different tanks and 2nd stages and you want to be breathing the correct mix at the correct depth.