Originally Posted by The Meat
Wouldn't used strings have it's elasticity stripped away from them, making them not loose as much tension as new strings? I always thought used strings would be like pre-stretched strings.
Not exactly. The tension loss of poly is not simply the poly losing elasticity. It is, as you can imagine, the string simply being stretched out due to the force you're applying it. Once you stretch it out so much to the point that it can't return to its original shape, then you've taken the elasticity out of it and the poly is dead. Now, that seems as though it's what you want, yes? No, because you are now re-applying a load to the strings by restringing them which is weakening the string rather than making it stronger. The tension loss now is going to be far more linear, but it is definitely guaranteed.
Think about it like this. If you want to keep something closed, then you need something either incredibly strong, or, something which can sustain a load elastically. If you're trying to keep the trunk of your car closed, then you could either use cord to tie it closed (the former case) OR you could use bungee cords (the latter). Naturally, like with tennis string, both cords are made of the same material (say, Nylon), only one is elastic. Now, the former is inherently going to be stiffer, but you rarely see this in favor of bungee cord. The reason is because since the bungee cord is elastic and resilient, it can store energy in it which increases the strength of the length of cord as long as you don't push it to fatigue and then failure. That length here is the crucial bit. The length of elastic cord needed is going to be much shorter than the equivalent rope needed to keep your trunk closed.
Likewise, a set of new string is stronger than the same 40ft length of dead string. The moment you apply a force to the fixed cord on the trunk (say, you hit a pothole and the trunk tries to fly open), you're going to damage it. With the elastic one, it will stretch and then return to its original state. With your tennis string, you hit a ball and the new string is going to stretch and then become shorter again due to its resiliency. With the dead string, you're stretching it out and it's not getting shorter. It's going to stay at that new longer and weaker state.
Hopefully that makes sense, I wasn't really too sure how to word that.