From the very beginning of polyester based string(s) the manufacturers advised stringers to reduce tensions by 5 to 10 percent, and some still do. You would be correct if you thought nobody did this back then! It was just too much to consider that kind of “softness”!
Today, however, tensions are going down and going down significantly! I pulled up some data of a pro tournament I did in 1992 and found that then Polystar users were still requesting 60 to 65 as their preferred tension setting, even in Head Prestige Mid 18×20 racquets! Now it is perfectly reasonable to use tensions settings as low as 40 pounds (18.14kg).
My recent experience is 44 to 54 pounds, depending on the racquet and what polyester based sting is being installed, is typical. But why?
One reason is we have been brought up on “reference tension” instead of “string bed stiffness” which can be different numbers. Another reason is that polyester based string was, and in some case still, not very stable. This means tension loss is quick. Of course polyester based string can be an arm, wrist, and shoulder irritant at high tensions.
Even at lower tensions some polyester based string exhibits a “stiffing up” as the ball impact force increases. That is, the harder the ball is hit the stiffer the string becomes. Here is the dichotomy…because polyester based string have low elongation, therefore, low power potential, the swing, and impact, must be harder to get the ball to go as far as you want, especially when freshly strung.
Lower tensions, (SBS), can help in the production of “spin” through compliance of the string to the ball. String movement and “snap back” may contribute a little to “spin” but it is difficult to quantify. To get the ball to spin the “slippage” of the ball on the string must be minimized, so reducing SBS is one way to achieve this.
The bottom line is that reference tension should be dependent on string, string pattern, player arm issues, and, of course, durability issues. A 16 x 16 string pattern will be better at a higher reference tension than a 18 x 20 string pattern. I have been working on a string that is much better at higher tensions!