Matt Waldman’s Rookie Scouting Portfolio showcases the elements of an effective stiff-arm, which 2019 NFL Draft prospect Devin Singletary delivers on a Marshall defender.  The Heisman. The Flipper. The Forearm Shiver. The Jab. The Bolo Punch. The Swipe. The Yank. The Head-Tilter. There’s a wide variety of effective stiff-arms. Although some are power moves, it’s…