What is a "top cap" on a climber's haul bag?

Question

What is a "top cap" on a climber's haul bag and how does it work? In Mountaineering: The Freedom of the Hills (and also at neilhopkins.us/mountaineering-guide-2/aid-climbing-equipment) is

A top cap to the haul bag is a good idea to protect the knot connecting the sack to the haul line and help reduce snagging problems while hauling.

but there is no diagram. I can imagine what it looks like because of

An effective top cap can be readily fashioned from an empty 2-liter plastic bottle.

Is the sequence like this, from bottom going up: haul sac, strap, carabiner, knot, (upright) "top cap", then haul rope, with the rope going through the small opening of the bottle?

Answer 1

That's exactly what it is, a top cap or knot protector is simply there to help protect your knot on the swivel from getting abraded against the rock while you're up-hauling:

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