Juice work is a gambling jargon for one of the most sophisticated types of marked cards. Although the cards are marked so that the cheat
can spot them from across the table, the marks only become visible when the cheat adjusts his sight, so that he throws his vision out of focus.
These types of marked cards are commonly used to cheat at poker and blackjack.
The image above shows a simulation of what the work would look
like, when the cheat throws his vision out of focus. In this case
the mark is a line at the edge of the playing card. This particular
type of work is called "line juice." The classic work,
however, consists of dime-size dots on most of the cards, and thick
lines running across the entire backs of a couple of cards (usually
the deuces and treys).