Just because I like it. This is obscure. We can ignore the upper section as this is quite specific to T cells (which the paper is all about). What I love is the consideration that mitochondrial glycerol-3-phosphate dehydrogenase (marked as GPD2), sitting on the outer surface of the mitochondria, is very likely to be driving reverse electron flow through complex I to generate free radicals. Something I would avoid, personally (except perhaps in my T cells).
The bit I love is the big red outline arrow from QH2 going to the left towards complex I.
The whole mechanism and specific purpose here is linked to activation of the drive for T cells to divide, a free radical mediated phenomenon through NF-kappaB. What interests me at the moment is what might happen to a cell which cannot divide when mG3Pdh is driven, say by hyperglycaemia acting on a neuron... Lots of papers to wade through on this.