A buildup of NADH relative to NAD+ slows down Krebs cycle enzymes that rely on NAD+.

“and when we then go back to the kreb cycle or TCA cycle this is going to slow down some key enzymes in that cycle that slow down the rate that at which that cycle happens there's three different enzymes that rely on uh NAD to function and so when we have levels of Ned relative to nadh it slows those down so one of the main ones here is going to be isocitrate dehydrogenase”