Crank refers to the speed at which a camera is recording. For slow motion shots, cameras are often over cranked, so that when played back at the regular frame rate, action plays out slower than real-time, without the jittery artifacts of simply slowing down regular footage.
With the advent of optical flow technology, some degree of under- cranking and over-cranking can be performed in post, artificially speeding up or slowing down the action.