Crystalline structures within the spider silk protein absorbed the maximum strain and broke, leaving the larger web intact. For example, a struggling insect would not break the whole web, but only those silk strands in contact with the insect. And after a local thread or two broke, the overall web strength increased! The study authors wrote that “the ultimate load capacity increased by 3-10% with the introduction of defects.”1 It’s as though the web was designed to anticipate breaks.
"A stabilimentum may be a single zigzag line, a combination of lines, or even a spiral whorl in the web's center. A number of spiders weave stabilimenta into their webs, most notably orb weavers in the genus Argiope."