I discovered this by accident as I was playing with my boyfriend. He tried to draw blood from himself to give a transfusion and revive me but accidentally drew the blood from me, then with nothing else to use it for, gave the blood back to me. I woke up, not for very long, a few minutes and I passed out again. I suspect it was a result of blood being possible to collect if the target's blood level is below the amount of blood taken/given by a transfusion, creating blood from nothing, but I do not know what my blood level was when this took place.