"detached arm" or "separated from shoulder"
"arm as projectile/missile"
"character's shoulder showing empty socket"
"arm flying independently"
Key phrases to avoid:
Anything mentioning "extending" or "reaching" (triggers Iron Man stretchy arm)
"rocket punch" (often keeps the arm attached)
For animation specifically, describing it as two frames can help: "before: arm attached, after: arm launched mid-air, body recoiling backward"