Man there's as many answers to this question as which cooker is best. There seems to be no definitive answer. I just did one in my egg indirect heat and did it fat up. I'll know in a little while if I like it or not.
Here's how I look at it. The heat goes up to the plate setter then reflects off of it to the sides where the gaps are. As there is not much room here the heat goes to the top of the egg following the shape of it, once at the top it swirls back down onto your meat. Who knows? That seemed logical to me so thats the way I went.
X-Large Big Green Egg