When there isn't enough oxygen for Oxidative Phosphorylation to occur, anaerobic respiration occurs. You can't produce ATP across the inner of the mitochondrial membrane or in the Krebs cycle if you don't have Oxidative Phosphorylation. As a result, the yeast employs anaerobic respiration to keep Glycolysis running, resulting in 4 ATP molecules (Net: 2) each Glucose molecule that is converted to Pyruvate.
Allowing NADH to lose hydrogen allows it to be converted to NAD, which can then be utilized to oxidize glucose to pyruvate, which produces ATP, and so on. This is best illustrated in a diagram, in my opinion.
