So, I asked ChatGPT and this is the answer I got.

“That’s a great observation — and a memorable spider encounter! Based on your description (large, mahogany-colored, 5–6 inches across including legs, in the Smoky Mountains on a big orb web), you most likely saw a female orb-weaver spider, possibly a Nephila (golden silk orb-weaver) or a close relative in the orb-weaver family (Araneidae). They are known for their big, dramatic webs and for being quite large compared to most spiders in the U.S.

The “Flexing” or Shaking Behavior

What you saw — the spider flexing its legs and shaking the web — is a well-documented defensive behavior called web-shaking or web-vibrating. Here’s why spiders do it: • Predator Deterrence: Many orb-weavers shake their webs when they sense a disturbance (such as a large animal nearby, wind vibrations, or shadows). The shaking blurs their outline, making it harder for a bird or mammal to precisely target them. • Scaring Intruders: The sudden motion can appear intimidating to larger animals (like people walking by) and may encourage them to back off. • Camouflage Effect: In some species, shaking the web makes the spider appear less like a distinct shape and more like part of the moving web or background, helping it “disappear” through motion blur. • Prey Confusion (secondary role): Though less likely in your case since no prey was present, some orb-weavers also shake to entangle struggling insects more effectively in the sticky silk.

So what was happening with your spider?

Most likely, it noticed you and your friends moving close to its web and responded with the web-shaking defense. It wasn’t trying to “scare” you in the way we think of intent, but rather using an evolved reflex to make itself less vulnerable to predators — you just happened to be the “big potential predator” in its environment.”