How to make a logo shine that moves.

I see this effect used all the time. You've got a logo over a background, and a light effect goes over the logo (perhaps from left to right) and makes it look like it's shining or reflecting.

How do you accomplish this? I've tried playing with lens flare and spotlight in FCP, but it also illuminates and exposes the alpha channel too.

Is this a motion effect? I have no idea...but it's so commonplace that I just know it's got to be simple to do. Please advise.

Thanks.