You can add this fusion only to a weapon that has the thrown weapon special property. A returning weapon flies through the air back to you after you make a thrown ranged attack with it. It returns to you just before your next turn (and is therefore ready to use again in that turn). Catching the weapon when it comes back takes no action. If you can’t catch it, or if you have moved since throwing it, the weapon drops to the ground in the square from which you threw it.