This is a Defence-Against-Projectile move. It looks simple but is actually highly technical. If the arm is flapped too fast or too slow, it has little impact. 10 beats per second is the optimal. Anything under 8 is too slow, and over 12 too fast. When deployed properly, the air compression around the arm creates an impermeable vortex, through which nothing can pass. So for example if a unit of Micro-scallywags hits you with a projectile attack of Instant Wrap Compound, Move 144 would be ideal.