Originally Posted by Povl Carstensen
Poly strings give less speed, but not less spin than other types of strings. So they have a higher spin to speed ratio, that is the real difference. You can hit harder, get more spin but not more speed (relatively), which basically means more control.
Just my theory but I rather like it.
It seems to work at even very low tensions where BP's deformation theory is perhaps not so much in play.
Edit: Oh, I see now that that is included in the link above, which seems quite comprehensive.
That's because even at very low tensions, a poly stringbed will still be stiffer than a multi stringbed at higher tensions. Thus, the ball will still flatten more against a poly stringbed at low tensions. And, yes, the low power of poly will also allow the player to swing more aggressively at the ball, which will of course, put more spin on the ball because you're swinging faster.