Baseball/Softball Bat Air Resistance Power Fin Swing Trainer (Replace Bat Weights that Harm Swings) Baseball/Softball Power Fin Swing Trainer by Markwort. 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 upset 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. Fits all baseball and softball bats (Little League Baseball & Softball 2 1/4, Big Barrel 2 5/8 & 2 3/4, Wood, Metal, etc)