[Mike C]

It means what it says. Like a limited slip, the no-slip or no-spin, works automatically- there is no external lever or button that needs to be engaged. It is the fore-runner to the Detroit Locker. Jack it up on the pumpkin and put it in neutural, NOT RUNNING, then rotate the tire forwards or backwards and if the opposite side turns in the same direction it is a no-spin. If it rotates opposite, it is an open diff.

I have a '70 GMC parts truck that was factory 4.57 and no spin (Dana 60) but when I pulled the cover off I found 3.73 open. So you never know exactly what your getting with an old vehicle.