Originally Posted by BreakPoint
Poly strings are stiff = stiff stringbed = more ball compression = more of ball's surface area biting into more strings = more rotational force imparted to ball when brushing up = more spin.
While I agree poly strings are stiff, it's not the inherent stiffness that creates the spin. I can string natural gut at 90 pounds and still not get a 'spin' friendly frame.
Look at Fed and Nadal, both who string in the low 50s and high 40's respectively. They have the two heaviest laden forehands on the tour. Do you think their string beds are harder then a multi done at 58lbs. As a stringer, I guarantee no.
Poly's are dead strings, that means they aren't very elastic. They absorb pace like a wet dish cloth. This helps reverse the incoming spin and increase dwell time. This combined with a vertical-friendly swing = lots of spin.