Power Weapons are weapons that spawn in floor platforms on multiplayer maps. A couple of examples would be the
Rocket Launcher and the
Sniper Rifle. Your AI will tell you when do they spawn, and you will see where they are in your HUD.
To drop a weapon, you have to equip it (have it in your hands) and hold the button. In order to unlock the achievement, description-wise, then an ally would have to pick up the weapon you just dropped.
As far as I can tell, you can pick up one of these weapons (
button), drop it (
button) and pick it up again (
button); the achievement unlocked for me that way, which is so dumb because I'm guessing as far the game is concerned YOU do count as one of your allies because you are always in the same team as you.
Some of you guys in the comments section say that
SWAPPING weapons does not unlock the achievement. Swapping is just picking up a weapon in the floor and dropping the one you have already in your hands AT THE SAME TIME (
button when you are near a weapon in the floor and you already have two weapons in your inventory).
You have to drop the Power Weapon first, then pick it up.