PolyTour Fire is designed for big hitters looking for lots of control, spin potential and durability. A firm monofilament polyester offering control. It still provides a surprising amount of feel and does not transmit too much shock and vibration. Yonex uses its Silicone Oil Infused Filament (SIF) process to manufacture this string. SIF reduces friction which increases snap back and spin potential. PolyTour Fire also offers above average tension maintenance compared to other strings in this category.