When striping your lawn, the weight of the roller pushes the grass in one direction or the other. Because the blades are leaning opposite directions when you finish, the light reflects off of them differently, hence giving the appearance of stripes. Just like when you vacuum your carpet. Now, I didn't make a roller, for reasons aforementioned, but I decided to just take some weights, and duct tape them to the flap on the back of my lawnmower. If I wasn't so lazy, I would have experimented with different weights, and see how that affected the stripes, or how hard it was to push the mower, but I am, so I just went with what I had laying around.