The process of transferring reducing equivalents from NADH in the cytosol into the mitochondria via glycerol-3-phosphate. Cytosolic glycerol-3-phosphate dehydrogenase uses NADH to convert dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate (G3P) in the cytosol; G3P then diffuses into the mitochondria where mitochondrial glycerol-3-phosphate dehydrogenase uses FAD to convert G3P back to DHAP; the electrons on the reduced FADH2 are then available for use in the electron transport chain, and DHAP returns to the cytosol to complete the cycle.