The jist of it is to grip at the base with one hand and apply pressure. With the other hand grip just below the head and jelq towards the base. At least this is how I understood it. I’ve never done these.
How would that even work when I thought the point of a jelq was to force all the blood up into the head to make it increase outward (length). I would think a reverse would just push it right back into your body instead of being trapped in the dick