Players with long, fast strokes should love the Yonex Poly Tour Fire tennis string is a red and round monofilament in polyester. This new Silicone oil Infused Filament (SIF) technology - monofilament infused with Silicone oil - allows big hitters to experience ultimate spin from intense snapback speeds. It will keep your biggest strokes inside the lines. Stretched under controlled temperature, YONEX strings deliver greater durability, reduced fatigue, and consistent repulsion. Greater snapback produces a powerful spin.