It’s awesome stuff, as this video explains.
Also recommended if you want to learn more: this book, Spider Silk: Evolution and 400 Million Years of Spinning, Waiting, Snagging, and Mating. There’s so much coolness in spider biology, I’m tempted to offer an elective in the subject…except that I think it might be too narrowly focused for our curriculum, and about half the students would refuse to go anywhere near it.