Penetration explanation
In a multiple deck shoe, after shuffling, a player is asked to place the yellow cut card somewhere into the multiple decks. A player then inserts the yellow cut card, cutting the multiple decks. (the player must place the yellow cut card so that at least one deck is cut away).
THEN, the dealer picks up all the decks (4, 6 or 8) places the decks against the shoe, pats the decks down, pulls the yellow cut card from where the customer placed it, then carefully, looking at the notches on the shoe, places the yellow cut card AT SOME POINT into the multiple decks, where it will remain for the entire shoe. This move, where the dealer places the yellow cut card into the multiple decks - AFTER the player makes his/her cut - is when "PENETRATION" takes place.
All cards behind where the dealer finally places the yellow cut card are DEAD CARDS and are not played during the shoe. The only exception is if the players are still playing when the yellow cut card finally pops up at the end of the shoe and more cards are needed, the dealer can then use several cards behind the yellow cut card to complete the players' hands.
To BE ABLE TO ESTIMATE/DETERMINE how many decks are left "dead" - or cut away (by the dealer's FINAL CUT CARD PLACMENT) from the cards that will be dealt in the shoe, go to the casino gift shop and buy a few regulation decks; then pile these decks, one at a time atop each other, in an effort to be able to estimate ("eyeball") the number of decks the dealer cuts away from the shoe when the dealer makes the last/FINAL cut with the yellow cut card -- RIGHT BEFORE THE DEALER PICKS UP ALL THE CARDS AND PLACES THEM INTO THE SHOE.
In an 8-deck game, the dealer usually cuts two full decks away so that 104 (52 + 52) cards are never seen or played in that 8-deck shoe. In a 6-deck game, the dealer generally cuts one and one-half decks away, so that the players never see 52 cards + 26 cards for a total of 78 cards in the six-deck shoe. In a 4-deck shoe, the dealer usually cuts away about one deck, so the players never see 52 cards out of the four-deck shoe.
The problem is that the cards cut away with the dealer's final cut and unseen by the players could possibly contain many tens, putting the player at a huge disadvantage in getting and blackjacks and tens when doubling down. Also with many 10's out of play because of the deep cut by the dealer, the dealer will break less often because he/she will get smaller cards when hitting instead of "breaking" tens .... M O N K E E!!!!!!!!
Questions that are often asked toward the end of a multiple blackjack shoe: Where are all the tens? What happened to all the tens? The answer: They're all buried behind the yellow cut card and no one will ever see them or get a chance to play them. Of course, the reverse is also possible, i.e., similarly, many small cards could possibly be buried behind the dealer's yellow cut card and out of play.
Confusing? Knowledge of this one small factor is among the many factors that separates expert card counters from "basic strategy" players.