If you use Defense Curl before using the move Rollout, the power of Rollout will be doubled. That means that instead of having a starting Base Power of 30, it will start off as a Base Power of 60, and then doubling to a Power of 960, instead of the original 480. This effect cannot be Baton Passed even if you have a Defense Boost passed* and this concept also works with Ice Ball, the Ice Type Rollout.
*Even if you Baton Pass Defense, you will not get the boost. You get the boost from Defense Curl only because of the fact that you are curling, making Rollout an easier move to use.