Stab forward slower than the Ubersaw taunt, then trigger the Inferno Blast. All angle backstab on hit, regular Inferno Blast if the tip is in empty air, Inferno Blast Explosion if the tip is in a target (fireball and gib ball if the tip was in a human or metal fragment ball if the tip was in a building or robot).
Inferno Blast (M2): Air Blast that burns and afterburns.