There are many ways to play deviations in blackjack, but the most common way to use them is as a double up option. When you hit on a 10 or an Ace and have a two or more cards that total 13 or more, you can choose to hit and then stand, or hit and then take another card.
This is called a Deviation Hit.
If you choose to hit and take another card, your hand becomes a three card hand with an Ace and two other cards. If the dealer has a 10 or an Ace in their hand, they will automatically stand, regardless of what you would have done otherwise.
Another popular deviation is the Split Hit. This is when you hit on a 10 or an Ace and have one card that totals between 9 and 11.
In this case, you can either stand or take another card.
The Split Hit is especially useful if the dealer has two cards that total between 9 and 11. In this case, you will end up with two hands: one with an Ace and one with two other cards.
You can then choose which hand to hold as your original hand or switch them both around.
Finally, there is the Undercut. This is when you hit on a 10 or an Ace but have one card that totals between 8 and 10.