This is Yonex's highest end multifilament string. It helps generate power while maintaining great durability thanks to the FRF (Fusion of Resin Filament) production process. The string has greater elasticity and feels solid due to the gaps between the filaments being filled with a new flexible material called Fortimo. WTA star Naomi Osaka plays with Yonex Rexis in the crosses of her racquet giving herself a little extra power.