For example, a stiff, less potent string might enable you to swing harder, thus causing you to secure more power. In our tests, series stiffness is the measured value acquired by changing one piece of string in a strain of 52 lbs. A string that is stiffer than another at 60 pounds will also be stiffer at 50 pounds. TW's string testing data allow you to compare strings based on those important measurements of string behavior. Polyester strings tend to be the stiffest and lowest-powered kind of series, but they do provide the most control, spin and durability. Greater stiffness usually means that the string won't divert up to contact, leading to more control, less power and less relaxation. This leads to less power because the higher power crushes the ball more and loses energy (balls are intended to lose 45 percent of the energy used to deform them). Longer live time doesn't, contrary to popular view, allow you time to guide the ball.