Rope gripping rope
Why Does a Knot Hold?
Nothing in a knot is glued, clipped or fastened. It holds because rope pressed hard against rope will not slide — and pulling on it presses it harder.
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Step 1 of 5
A straight rope has nothing holding it.
Pull on a rope threaded through a ring and it slides straight out again. Rope is smooth enough that merely lying against something is nowhere near enough to stop it.
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Step 2 of 5
Grip grows with how hard the surfaces are squeezed.
Two things slide easily when they rest together lightly and refuse to slide when they are pressed hard together. A knot's whole job is turning your pull into that pressing.
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Step 3 of 5
Pulling on the rope is what tightens the knot.
The load does not fight the knot, it feeds it. Every kilogram hung on the end is pressed into the crossings, so the harder you pull the better the knot grips back.
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Step 4 of 5
Each turn multiplies what came before it.
Which is why a rope around a post hardly needs holding. Grip does not simply add up turn by turn. Every wrap holds back part of the pull, so the next wrap has only the remainder to deal with, and three turns hold many times what one does.
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Step 5 of 5
A knot you can undo has one loose loop left in it.
A bow is an ordinary knot with a folded loop parked in the middle. Pull the loose end, that fold slips out, every crossing loses its pressure at once and the whole thing falls apart.
It is also why a lace made of slippery round cord comes undone all day and a flat cotton one stays tied: less grip at every crossing.
The short version
A knot holds because bending rope back against itself turns your pull into pressure at the crossings, and rope squeezed against rope will not slide.
Try it yourself
Wrap a string twice around a pencil and pull one end hard. Feel how little you need to hold at the other end to stop it moving.