It's when someone follows to get a follow back, followed by an unfollow that leaves the followed unfollowed.
Similar to the American Football / Basketball move "juking" - when someone thinks you're going left or right and you move the other way, leaving the pursuer empty handed.
Mike follows Lisa, Lisa follows back - Shortly after, Mike quietly unfollows Lisa, leaving his follower count one higher and Lisa thinking she has a follower she doesn't.
"Dude, I was just twitter juking and I juked about 300 people... they followed back and I left them high and dry... "