How does this work?

As an example, let's say the negligent driver has purchased a $25,000/$50,000 auto liability insurance policy, and you have a policy that provides $100,000/$300,000 in UM coverage. Your injuries are serious. Your medical bills and lost wages are substantial. In your lawsuit against the negligent driver, a jury awards you $100,000 in damages. The defendant driver's insurance will pay you only $25,000, the maximum amount of liability coverage the defendant bought which can be paid to one person with respect to one accident. However, you can look to your own UM coverage to see if there is under insurance coverage available to you. Given the above facts, your insurance company would pay you $75,000 in underinsurance coverage.