spider zigzag

There is one of those large, brilliant yellow and black spiders in my garden- with a very interesting web. It has a zigzag right down the middle, like the arachnid was trying to make a zipper. 

The yellow garden spider.

Not sure why it makes the zigzag pattern. I guessed it was for adding strength to the web, my husband said maybe for camouflage- standing from one side it is hard to see the spider in the middle. Whatever reason, it's very cool!