Originally Posted by J011yroger
Two reasons actually.
Kevlar has no elasticity. Normal strings are better at absorbing and returning energy than the ball is, so stringing them looser allows them to absorb more of the impact, returning more of the impact. Kevlar is worse at absorbing and returning energy than the ball, so you want to string it tighter to make the ball absorb more of the impact.
The second reason is that because the Kevlar has no stretch to it, once it loosens up a bit it just flops around in the stringbed doing nothing, so when you string it loose it feels great for a little while, then turns to crap. By stringing the entire bed tighter, it hangs in there a bit longer because it is tighter to begin with, and has the tighter cross to back it up.
I also find this true. Also, since I started playing again this is the first winter I've had kevlar since I was 18 many moons ago. Cold didn't seem to bother me then but tight kevlar and temps in the 40s-50s makes it very jarring. I say string lower as the temps drop. I've found the 18g to hold tension a little better than the 17g, which is very weird.
See this for other kevlar etc info. The 17g loses something like 28lbs of tension.