NOOB! Diff question.

So i'm used to my differtial bein up in the air and spinning one tire forward, the other tends to go bckwards. well i had my dart in the air for some wheel cylinders, and i was turning the wheels, to see the other side do NOTHING. The shop teach mumbled something about non-slip somethin somethin. but was a lil busy. I've got a lil mini 7.25, as its originally a /6 273 car. So wat is up? Sate my curiousity.