Here is my guess. If you twiddle your thumbs in the screw-in direction, the heads move together at twice the rate that a head would move when screwed into a fix

Here is my guess. If you twiddle your thumbs in the screw-in direction, the heads move together at twice the rate that a head would move when screwed into a fixed thread. And in the screw-out direction, the heads would move apart at twice the rate of dissassembling a bolt from a fixed threaded hole.