Here's Rich's explanation from 2012:
"When methylfolate is made naturally in the cells, MTHFR does it, and it needs riboflavin. If you supply methylfolate as a supplement, then it feeds the methionine synthase reaction, which needs methionine synthase reductase to keep it going, and that in turn also requires riboflavin. If a person is deficient in riboflavin, it will block recovery when using the methylation protocols for these reasons as well as for other reasons, involving the energy metabolism, the neurotransmitters metabolism, and others."
"When methylfolate is made naturally in the cells, MTHFR does it, and it needs riboflavin. If you supply methylfolate as a supplement, then it feeds the methionine synthase reaction, which needs methionine synthase reductase to keep it going, and that in turn also requires riboflavin. If a person is deficient in riboflavin, it will block recovery when using the methylation protocols for these reasons as well as for other reasons, involving the energy metabolism, the neurotransmitters metabolism, and others."