The Harken 75mm Flip Flop Block w/Lockoff features a large 75mm sheave with ball bearings for smooth trimming and low friction, even under heavy loads. The key innovation lies in its hinged construction and its lockoff feature, with is healpful in stituations where you need to hold line tension without using a winch or cleat. The block itself is built with lightweight and durable 6061-T6 aluminum cheeks that pivot on fiber-reinforced plastic chocks. All components, including the sheave, ball bearings, and sideplates, are UV-stabilized with carbon blacking.