Counting cards is not dissimilar to counting gumballs. First, just as there were good gumballs, bad gumballs, and neutral gumballs, in blackjack there are good cards, bad cards, and neutral cards. And just as we assigned a value to the different gumballs, we can also assign a value to each type of card. We then start with an initial running count and count through the deck as the cards are played. Once the running count reaches the key count. we know we generally have the advantage. When the running count is equal to the pivot point, we have a reliable estimate of the expectation.
There are two patterns in counting cards: during the hand and at the end of the hand. With the "during" pattern you count each card as you see it turned up. This way you have the accurate count at any point in time for the cards you have seen. The disadvantage is that you may sometimes get confused as to which cards you have not already counted and which should be skipped. This can easily happen when a player tosses his down cards on top of his hit cards after busting. You suddenly see a pile and can't remember which of them were up cards that you have already noted.
With the "end" pattern, you wait to count the cards until the round is complete, counting "during" only the cards that are in busted player hands (the dealer collects these before continuing to deal to the next player). This method of counting eliminates the confusion of which cards to count. The problem with counting the cards at the end of the hand is that you must count them quickly, as the dealer turns the hand over and scoops up the cards. It's easy to make a mistake counting this quickly. Another problem is in using the count to modify Basic Strategy. Without the current count, you will be making playing decisions based upon the condition of ,the deck many cards ago. With a full table, your count for playing strategy is likely to be dangerously inaccurate.