I'm having difficulty finding articles specific to the Leaf's motor, but there's a nice explanation for EV motors in general here:

http://www.diyelectriccar.com/forums/showthread.php/why-do-e...

Basically, the motor controller limits the current below its maximum so the amount of torque output will remain constant until it drops below the current limit. There's an explanation of different current limiting methods here:

http://homepage.cs.uiowa.edu/~jones/step/current.html

The Leaf's controller likely uses an active current limiting method, and there's a lot of advances in this technology right now. That's where a lot of the motor efficiency improvements are coming from.

Surprisingly, I can't find much good info on Wikipedia, but I'm probably not using the right search terms.