Social Security is a key part of your financial security in retirement, but many people leave valuable benefits on the table because they don't know about them. By knowing some obscure rules, you can earn more from Social Security.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, talks about a little-followed strategy that could be worth $10 billion annually to eligible Americans. As Dan notes, people are eligible to take spousal benefits when they reach full retirement age, giving them the chance to claim those benefits while delaying taking benefits on their own work history until as late as age 70. Doing so can add 32% to your own retirement benefit while letting you get money at an earlier age, with one study showing that the strategy can add 3% to your lifetime take from Social Security. Overall, that adds up to as much as $10 billion that the Social Security Administration could have to pay if more people used the strategy.