Franco had a great offensive line, and Rocky Bleier was not afraid to aggressively block for him as well. Franco was known to dot a lot of shuffling left and right waiting for just the right hole to open up.
I had an uncle, long since passed, who used to say if Franco had as many yards going forward as he did sideways, nobody would ever be close to him in career yardage :)
He definitely did shuffle when running, especially as he got older. But he and Rocky were the first pair of running backs to go over 1000 rushing yards in the same season.