Why do spiders jerk when disturbed?

I’ve noticed that a spider will jerk its web with quick sudden movements when you disturb them. A puff of breath will cause them to do it, or even a fairly large bug captured in the sticky strands of the web will make certain spiders jerk its entire web, as if attempting to dislodge the trapped insect. Does anyone know why a spider might do this? I’m looking for someone with a scientific view or actual knowledge on this phenomenon, I can come up with plenty of smart-ass responses myself.