You indicate to the dealer that you want to split.
Suppose you wager $10 and are dealt a pair of 8s. In addition to the Free Doubles, there are also Free Splits where the casino will pay for the secondary bet on splits, meaning the dealer will place a Free Bet lammer on the second hand of a split. You can still double down on a soft hand, but you do not get the free double.
Essentially, you risked $10 on the hand with the opportunity of winning $20.