"We have a legal definition of brain death: NO more brain activity."
You are badly misinformed. You must think that "brain death" is determined by some machine that measures waves and when the waves stop you are dead. That is science fiction, not science. Read up on it. Yours is not a medical definition of "brain death"
If there is no brain activity, no machine can keep you "alive" in a body that is functioning in any way. The bodies of people who are supposedly "brain dead" do all kinds of functions. The blood that circulates is not hand-walked through your body. The "nerve" lines between the brain, heart and lungs are working (alive).
If what you say was true we could go take bodies out of the ground and make their hearts beat and their lungs breathe, we cannot.
Give your organs if you wish, but know that they will be taking them from you, a person still alive.
Michael is from Germany - he's talking about German laws.
It is a a definition given by the law to make sure that only dead people will donate organs. The machine tells you whether a patient is brain dead or not. This is not science fiction, but daily life for organ transplantations.