The whole point of having a strategy for online blackjack is to win more. And winning more means more money. There is one play in online blackjack that can increase your profit faster. It should also be a big part of your online blackjack strategy as well.
Doubling down is one of the basic plays in blackjack. It is when you double the amount of your original wager and only receive one more card. If you win the round you win double the amount you would have normally won because you doubled your original wager.
It has the same potential of winning a round as two rounds of blackjack. In effect, if you double down at the right moments (read: play according to basic strategy) you can earn twice as much money as you normally would. That is what makes doubling down such a strong play in online blackjack.
If you can double down when it is most advantageous (again, read: play according to basic strategy) you can reduce the houses edge by 1.6%. This is all because you are winning twice as much money in one hand as you normally would.