A rule that lets you double down on hands after a split.
Good for the player; it lowers the house edge by about 0.14%. With DAS, you split 4-4 and 6-6 against the dealer's weak cards more often. One of the rules to check at the table.
At the table
After splitting 8,8, if the first hand draws a 3 for 11, a table with DAS lets the player double on it. Without DAS, that hand can only hit or stand.