[jome]

It's not a black and white thing. Different shades of gray, I'd say. Samba is just the early stages of BO...

It's usually called a BO when someone is clearly "out" for a period. Totally unresponsive and memory loss. But a tough samba doesn't look much different.

Surface bo's are sometimes hard to differentiate. But of course much clearer are BOs that happen in dynamic or deep diving - the person simply stops moving or loses control completely.

About the 4 minute rule - yes, 4 minutes without oxygen is dangerous for the brain. However, the brain is not "without oxygen" when you hold your breath. For an experienced freediver doing static the oxygen levels in the blood may not even drop noticeably in 4 minutes (I once tried this with an oxymeter and at about 3:45 it started to drop from 99% saturation). After it does start to drop, there are several compensatory mechanisms that prolong the period before the brain starts to suffer from hypoxia (mainly, increasing the bloodflow to the brain). But pushing your self to Samba or BO is not wise. We've seen that happen and these people are not quite brain dead, but it's safe to assume it's not good.

But after the BO, every second that you don't breathe is worse, you'd want to be rescued as swiftly as possible. After some minutes, serious brain damage is thought to occur.