Aeris Velocity X3 fins The Velocity X3’s angled blade features a large rubber center section flanked by rubber-reinforced thermoplastic side rails that tie into the foot pocket to add rigidity and pump up the kicking power. The center section has a series of Power Enhancing Vents (PEVs) that give the water pushed by the fin a place to go other than in front of the other fin, increasing kicking efficiency. The combined effect of rigid side rails and a pliable, vented center panel causes the blade to behave similarly to a split fin, with good blade flex and lots of power. Please note these fins come with a rubber fin strap!