[Uppercut]

One of his deliveries could conceivably have been a cutter. Bowlers can use their index and ring fingers to spin the ball from leg to off. It's easy enough to control and the ball sometimes cuts quite a lot off the pitch for a seamer.

In addition to this, one can have a lot of variations- a quicker ball, an inswinger or outswinger, an off-cutter. But the leg-cutter would have been the one that noone else bowled, hence the one everyone remembered. In particular, it could have been one or two high-profile incidents where the ball broke heavily off the pitch that gave a lot of onlookers the impression of a fast leg-break bowler.

I'm only speculating, of course. It's all anyone really can do, given the information available.