Yonex Poly Tour Fire tennis string is designed for players with medium-to-fast swing speeds looking for added power and spin. This new Silicone oil Infused Filament (SIF*) technology--monofilament infused with Silicone oil--allows big hitters to experience ultimate spin from intense snapback speeds. Greater snapback produces a powerful spin. SIF Technology with unique pentagonal structure allows the string to move faster than conventional models, increasing spin. *SIF technology is developed in cooperation with NANODAX