I have a number of books on gambling scams and cheating, and several mention the “juice” method of marking cards as being a nearly perfect technique. However, most were written at the time the technique was in first wide use and thus highly confidential among grifters and sharps.
Googling around now, I see you can buy juice and an instruction booklet for around $50.
Does anyone know what “juice” is and how it’s used to mark cards?
From what I understand, a “juice deck” is marked in such a way that you only see the mark if your eyes are unfocused.
edited to add: In other words, it isn’t the deck that is juiced-it’s the sharp! 
Well, you know the juice. Prob’ly mark the deck with a menorah or a dreidl or somesuch.
What ? What did I say ?
Right, I know how it works in a functional sense; what I want to know is (1) how does it work in a technical sense and (2) what is the “juice” compound?