Interesting library. I will have to look at this some more in the future. Basically it will allow you to control 4 motors at a time with PWM (and direction). It looks like its using an L293D to power the motor under the hood.
http://dangerousprototypes.com/2013/11/25/driving-a-dc-motor-using-software-pwm-with-avr-atmega/
http://davidegironi.blogspot.com/2013/11/driving-dc-motor-using-software-pwm.html#.UpTaTHVDtu5
Older reference here
http://davidegironi.blogspot.it/2013/03/controlling-dc-motor-speed-and_6.html#.UpTafnVDtu4