Like those toddlers who get stuck in a fence or alikes. How can they put their head in but once it’s past it’s impossible to get it out again. Or even a ring, a couple of minutes after putting it in it’s harder to get it out
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Okay, so take an arrow for example here… And bear with me but this will make sense in a minute. The shape allows for the arrow (diamond shaped or triangular) to puncture flesh, but the pressure from the body closes around the arrowhead and makes it very bad to pull out, causing more damage as it goes in.
Now, you want to think about the shape of the head and realised that your ears make a similar shape as an arrow. You can go forward, but not back as your ears aren’t shaped that way. You put your head through the bars, you might not get it out.
Knuckles have a similar effect with their shape, and skin basically smoothes out to allow the ring to pass the knuckle bone, then somewhat rolls up against the knuckle to stop the ring going back.
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