Sabin will eventually learn the Bum Rush technique on his own at Level 70, but you can expedite the process by visiting Duncan's Cabin in the World of Ruin.
The Bum Rush is the only magic Blitz that ignores Mag.Def, making it quite powerful—more than triple the damage of Air Blade against an average Mag.Def. Unless you've seriously pumped up Sabin's Vigor, Bum Rush is his most powerful single-target attack. (Even at max Vigor and starting Mag.Pwr, Suplex doesn't overtake Bum Rush until Level 47!)