Sigh, torque is a force about an axis. Plain and simple. When you use a wrench on a bolt, the rotation of the wrench about the bolt (the axis) is a result of torque. Torque is defined as tau = rFsin(theta), where tau is torque, r is the displacement, F is the scalar of the force, and theta is the angle of the applied force. In the wrench, it's why when you push perpendicular (90*) on the wrench you get maximized torque for your distance (sin90*=1), and the farther out your hand is on the wrench the easier it is to rotate the wrench (r gets larger).
On a disc at the end of the hit is when the hand is torquing the disc. Also worth noting is that for our intents and purposes, r is constant. The disc is pinched between the rip finger and the thumb (pad) and from our frame of reference is rotating around this point. From the disc's inertial frame of reference, the rip point is applying a force back toward the body, and this results in the torque that gives the disc its spin.
I hope this helps, but I've been watching Suits and typing this in the commercials so there may be an error or two. This is just meant to help give a fundamental understanding of the forces at work. It'll take some more time and some reacquainting to work it out more. I strongly recommend checking out the thread that Dan Ensor linked to (and thank you for that) especially the first page.