If the dealer’s upcard is an Ace, the player is offered the option of taking Insurance before the dealer checks his ‘hole card’.

The player who takes Insurance can bet an amount up to half his original bet.

Usually the Insurance bet is placed separately on a special portion of the table, marked as “Insurance Pays 2:1″.

Taking Insurance means betting that the dealer’s ‘hole card’ is a 10-value card, i.e. a 10, a Jack, a Queen or a King. Because the dealer’s upcard is an Ace, this means that the player is betting that the dealer was dealt a natural blackjack, i.e. a two-card 21 (a blackjack), and this bet by the player pays off 2:1 if it wins.

Short example:
The player bets $10, the cards are dealt, the player’s hand is 18, and the dealer shows an Ace.

The player takes Insurance by betting an additional amount of $5 (half of his original $10 bet). The dealer checks the hole card and sees that it’s a natural blackjack. The player loses his $10 bet on his hand, but he wins the insurance bet, so the player gets 2:1 on his $5 Insurance wager and receives $10 (on top of the $5 which is returned to him).
Note that the player came out even on that round (i.e. did not lose any money).

Conversely, a player may win his original bet and lose his Insurance bet. Let’s say we have the same situation as above. But this time the dealer’s hole card is not a ten, but a seven. In this case the player loses his $5 Insurance wager. But the player wins his $10 bet.
Note that the player made a net profit on that round.

Of course, a player may lose both his original bet and his Insurance bet (in the example above: if the dealer’s hole card is a nine).

Insurance is a bad bet for the player who has no estimation (e.g. through card counting) of the dealer’s ‘hole card’ because Insurance has a negative expected value for the player.

Insurance always pays 2:1, while the actual odds of a dealer having a blackjack are 2.25:1 depending on the number of decks being used.

Even players who have been dealt a two-card 21 (blackjack) should not take Insurance.

Why?

The player with the 21 accepts to be paid off at 1:1 instead of 3:2 when winning this hand.
Thus it is exactly the same thing as buying Insurance, losing the Insurance bet and getting paid 3:2 on the natural. (If the player with the natural refuses the offer of “even money”, and the dealer turns over his hole card to make a natural (a blackjack), it is a tie and the player’s bet is returned to him).

Using basic card counting at multiplayer BlackJack tabels insurance can make a profitable bet. Just count how likely it is that the dealer draws a ten card.