Markwort Power Fins Work against air resistance (similar to swinging a bat in a swimming pool). Build swing strength, increase hand/bat speed, develop stronger wrists. Batting donuts have been found to do more harm than good, as they disrupt your natural swing process and the weight distribution of the bat. Swing mechanics are not altered with Power Fins. Slip the fins over the barrel of the bat. One size fits all baseball and softball bats, little league, high school and college. Assembly instructions included. Weighs 14 oz.