BULLETPROOF X TILT, TOROLINE's most durable offering, features two co-poly tennis strings; one designed for the main strings and one designed for the cross strings. The main string (BULLETPROOF) is a seven sided shaped co-poly designed for spin, control and durability. The cross string (TILT) features a round profile coated with a slick surface. The slick surface allows the main strings to slide out of position, grip the ball and then snap back to generate massive spin.