Casinos use them to check for a blackjack in the event that the up card is an ace or a face card. The way it works is the dealer slides the cards into the slot, if the window shows a blank, it is not a blackjack. If the dealer has blackjack, it will show an ‘A’ or a 10-K (depending upon the up card). It was invented to prevent collusion between a dealer and player. When using this device, the dealer would only know if he/she has blackjack. If not, the dealer has no more information than the player (ie the dealer doesn’t know what he/she has if it’s not a blackjack). This also helps to speed up the game as a blackjack would end the round of dealing. Since the casino’s profitability is a function of how many hands are dealt, the dealer peek saves time by ending a hand as soon as it is determined to be a blackjack as opposed to dealing out all of the cards only to find out at the conclusion that he/she has a blackjack which would have ended the hand sooner— saving both time and cards remaining in the shoe (thus, allowing for more hands to be dealt).