How does that hinged jaw on a pole stay shut when you're pulling?

It's pure tension, no spring needed. I built one of these a few years back to fish wires through a ceiling void, and the whole trick is the geometry of the pivot. The jaw is a lever with the pivot somewhere in the middle, and the pull cord attaches on the short end of that lever. When you pull down on the pole, the cord pulls that short end, which squeezes the jaw shut. The harder the cord pulls, the harder the jaw clamps.

The clever part is that the thing you're grabbing pushes back through the jaw, which actually adds to the closing force instead of fighting it. So a heavier or more awkward load just makes it grip tighter. That's why it feels like there must be a spring in there, because the grip seems to self-adjust, but it's all just leverage and rope tension doing the work.

To release it, you swing the pole or flip it so the cord pulls on the long end of the lever instead, and now the same tension pries the jaw open. No moving parts except the pivot, which is why it's such a satisfying design. Mine was just a scrap of hardwood, a hinge screw for the pivot, and some paracord. One tip if you make your own: keep the cord attachment point close to the pivot on the closing side. If it's too far out, you lose mechanical advantage and you'll be pulling on the pole like a fool trying to grip something the tool could've held with a gentle tug. And wax the pivot or use a nyloc nut so it doesn't work loose mid-lift, since a dropped grabber at head height is no joke.

I'm skeptical about the idea that just pulling harder guarantees a tighter grip. While more tension obviously creates more force, there's a point where you've just reached the maximum structural capacity of the jaw material itself, or the fulcrum is positioned in such a way that you're just wasting energy past the optimal clutch point. I've found that cranking the pull too tight on these pole grips doesn't actually improve the grab once it's past a certain threshold.

Also, flipping the pole just to release it seems a bit clunky. It would be simpler to just use a manual release release pin or pin on the pivot itself, rather than relying on changing the entire geometry of the lever assembly just to open it.

I've used these things for grabbing fasteners on roof trusses. The key thing people miss is the effective grip area; a cheap, smooth piece of steel won't hold anything substantial. When I modify mine, I wrap the inner gripping surface with a high-friction rubber strip, ussually at least 1/4 inch thick.

That little change makes all the difference in load-bearing ability. I once had to haul a rusted lag bolt that weighed nearly 50 pounds, and without that rubber padding, the jaw would have just slipped under the stress. It really makes the difference between a casual grab and a high-tension hold.

Quick update - the tension explanation was correct, no spring needed. I acutally went with the grip improvement suggestion from one of the replies. Wrapping the internal gripping edges with a high-friction material completley stabilized the catch. It makes all the difference when dealing with slippery materials.