it's interesting to see that working the spring like that would soften it.

Normally I would say that working a metal of any type, would gradually harden it, making it more stiff and brittle. Though I would have said that would take a whole lot more cycles than could be applied by hand in a 10 minute period. Perhaps it's got something to do with the state the springs are in straight out of the factory?? I'm going to have to look this up. Applying the slippery plastic is a brilliant idea.