The real form of this phrase is similar to a masturbatory experience known as "The Stranger" (in which you sit on your hand until it is numb and masturbate so that it feels like someone else is doing it)
The only variance is that when doing a "Cactus Jack" you wait for partial numbness to restore until you feel the "Pins and needles" in your hand. You then masturbate and it feels like you're jacking off a real cactus.
Other Notes: It does not matter much if you reach climax during this as real cacti retain liquid and cannot orgasm.
Should you jack off a real cactus please seek immediate medical help.
John was giving himself a stranger in the closet when suddenly it turned into a god awful Cactus Jack situation when the feeling in his hand came back. Next time John will sit on both of his hands before an attempt.