The fat is a sacrificial shield from the heat and a lot of the fat is just going to cook away anyway. The meat side is where we want to protect and try to get that nice bark.
I have heard it argued that if you cook it fat side up, the fat will baste the meat and keep it moist. This sounds good but all the fat really does is run down the side and into the fire or water pan.
Fat side down really only relates to a direct heat smoker, ala. BGE, UDS, etc. If
using an indirect heat source it really doesn't matter, and generally I go fat side up
because it's easier to clean up and because it might just end up a tad more moist...
I've always done fat side down for the heat barrier but also my own personal theory be it right or wrong is the fat dripping down the side could "wash" some of your rub off. Whether that's true or not I dunno. Like everything in BBQ there is more than one way to skin a cat.
Fat side down on everything for me. It becomes my "skid plate" so I don't have to worry about jacking up the "pretty side" as I move meat around the pit. I also agree that it does help somewhat as a thermal barrier as well.
I was taught fat side up, but I started getting better results when I did fat side down, whether I'm cooking them in the Backwoods or Ole Hickory. Just seemed to have a better bark, and in comps, I went from the middle of the pack to consistently being in the top 5.
What fat?? I trim all my fat off because I love the bark and bark wont form on fat, IMO fat cap on top will just run off and not penetrate the meat anyway. I use water in my pan (WSM) so the bottom is protected from direct heat anyway.