wrinkly skin
My balls have churned only when there was plenty of skin, in agreemnet with your observation/premise.
I was ready to deep six the infra-red theory, but churning balls in a sack that isn’t experiencing a temperature differential doesn’t make much sense.
If the sack is exposed to a constant temperature, then the entire sack will expand or contract to reach equilibrium, and the balls will move during the change, but this isn’t churning. the correct size for the temperature is quickly found, and the ball movement will stop.varying temperature will cause the sack to constantly change size, but no-one is reporting hot then cold then hot in rapid order. Infrared heats only part, and even the side towards the source will experience shadows causing continuous changes that you wouldn’t be aware of. The same effect would happen with a low level draft. But it still seems to me that only part of the sack can be heated (or cooled) to get a churning vs simple movemnet. And cooling could easily result in up-tightness.
If i remember correctly, left and right foot size and hand sizes are actually different too. My right boots are both looser, indicating the right foot is shorter. Isn’t really winter time yet, so I haven’t got my gloves out yet.
Flip may have stumbled upon the true secret of the universe. Explaining them could make e=mc2 nothing but bland duck soup.