This might not be the "final say" but it is a deep look into the issue:
How to handle a freediver suffering from blackout
This "thesis" is part of my exam as an Aida Instructor
Sebastian
PS. Dont like the video. There are things missing and it might not be a good idea trying to save someone who is apparantly busy saving himself. Keep away from divers in distress, they might put you in trouble (ending up with TWO accidents). It might sound cynical, but wait until they have blacked out if they look paniced.
