It's done by making the grass blades lay over towards your viewpoint to create a darker stripe and away from your viewpoint for a lighter looking stipe. Normally done by a roller that follows the cutting deck, but there are some aftermarketers that sell a brush that attaches to the mower that gives the same result.

Here is a video that shows one way to make wide stripes in a lawn. On youtube, there are a number of videos on lawn striping, and there are different methods used to make stripes on a lawn.

http://www.youtube.com/watch?v=hcMD72YAv-I&feature=related

You need a mower with a roller, Simplicity tractors have a roller and can stripe quite well. I am sure there are others out there, I don't know of others though.

I have a Simplicity mower with the full length deck rollers. I LOVE making different patterns in my lawn! It's also better for the lawn if you cut it in different directions.

I believe there are aftermarket add-on's out there that are supposed to give you the same effect, but I have no experience with them.

BTW - my mower WILL NOT create stripes as bold as the ones you see in pro baseball parks or on golf courses. But it's close enough for me.