The basic effect
Every time a card is dealt and not returned, the cards that remain are different from what they were. If the remaining cards are no longer in the original proportions, then the probability of each possible next card changes slightly. The effect of that change on the player’s expected result is what is meant by card removal.
A full deck has 52 cards, with 16 of them worth ten (the tens, jacks, queens and kings) and 4 aces. If one ten is taken out, the chance that a random card from the remaining 51 is a ten falls from 16 in 52 to 15 in 51. Those fractions are about 30.8 percent and 29.4 percent. The chance dropped by a little under one and a half percentage points from a single removal.
Why a ten leaving usually hurts the player
Tens and aces matter for natural blackjacks, for doubling on strong totals and for the dealer’s finished hands. When tens are removed, the player has fewer chances to be dealt a natural and fewer big doubling wins. The dealer also has fewer chances to make a natural, but in net the loss of high cards generally lowers the player’s expected result.
This is the logic behind the idea that a shoe with fewer high cards remaining is less favourable. It is the same idea that makes high cards negative in tagging schemes.
Why a small card leaving usually helps the player
Small cards, particularly the twos through sixes, tend to help the dealer by letting the dealer reach a completed hand without busting. They also tend to make a player’s hit decisions less costly. When these small cards are removed, the remaining shoe has a larger share of high cards, and the player generally gains slightly. Dealer bust cards are a good example: the reason dealer upcards of two to six are weak is tied to how often a dealer starting there must draw into a bust.
The effects are not identical for all small cards. Different ranks have different strengths, and a few ranks, such as sevens, have close to no effect on average. This variation is why simple systems assign some ranks a value of zero and give others a plus or a minus.
Putting a size on it
The size of the effect from any single card is small, and it should not be exaggerated. As a rough illustration with invented round numbers: if removing one particular card shifted the player’s expected result by 0.3 percent of the bet, then removing ten such cards might shift it by about 3 percent, assuming the effects simply added. In reality, effects are approximately additive, which is why counting systems work at all, but the exact values depend on the rules and the number of decks.
The number of decks matters a lot. In a single deck, removing one card changes the proportions noticeably. In a six- or eight-deck shoe, the same removal is a much smaller fraction of what remains. This is also why the infinite deck simplification, which treats each draw as independent, is a reasonable approximation for many calculations in large shoes, though it ignores exactly the effect described here.
From single cards to composition
Beyond one card at a time, the overall composition of the remaining shoe matters. Decisions on specific hands can depend on which cards make up the player’s total, not only the total itself. Composition dependent play describes this: with the same total, the best action may differ slightly depending on which cards form it. These effects are tiny and are usually not worth worrying about in ordinary play, where basic strategy already captures nearly all of the correct decisions.
What this does and does not imply
Card removal is a statement about averages. It does not say the next card will be high or low, and it does not mean a streak of low cards is owed a streak of high ones. The shoe has no memory beyond the cards physically gone, and a change in odds is not a promise of results. A tilt in the player’s favour still leaves most of the risk of ordinary variance in place.
Blackjack carries a house edge in ordinary play, and uncertainty remains under any circumstances. Gamble only with money you can afford to lose, and contact a local support service if gambling is causing you difficulty.