Reading this old threard. Good discussion. 40% O2 at 40 meters is PPo2 of 2. No, that's not sport or recreational diving, and no certifying agency would say it is. We can't take one of two lines of the training manual and ignore the context of the rest of the guidance. Hopefully we all agree on that.
Divin'Papaw makes great points above about negative entries. There are times when negative entries are helpful. I've been on boat dives where due to rough seas and strong surface currents the captain has instructed us to enter negatiely so we can catch the down line quickly, but it does add risk. Even with the above mentioned checks, there's still potential for thing to go wrong; e.g., boat bounces when you step off and your yoke reg connection gets smacked and loses solid connection to the tank, mask pops off and you're disoriented,...
A few years back I read a report in the back of the DAN magazine about a guy on a boat dive with new gear who stepped off and went straight down. He wasn't trying to do a negative entry, but he did, and was found at the bottom (about 80 feet, I think) unresponsive. His air was off. You can imagine him finding it out as he's going down, exhales, and then nothing when he tries to inhale. Yes, it was a fatality. We all hope we'd handle a situation like that calmly and rationally; evalute our options, pick what's needed and do it (reach back and turn on air, ditch,...). But when a true life or death situation is occuring, so can an amygdala hyjack where rational thought goes away and you're just reacting in a very primative way to try and stay alive. That's why we think through scenarios and practice things like reaching our air valve, ditch and don, ... knowing what to do to stay alive and stay in control. But many police officers who train regulary with their firearms have it happen when a true lfe or death situation happens. So, it can happen even if you think you can handle it/ have prepared. So if I don't have to do a negative entry, I don't. If I decide I want to practice it, I tell my buddy, have them go in first and watch as I come in.
Just got back from a week of easy diving in Roatan. Four boat dives a day. Captain and DM didn't require you to signal all good before descending, so almost everyone was entering negatively. It was just easier than having some air in your vest/ wing, dumping, and then swimming down. I love this sport, but I know I can't breathe water. I think about that guy (and his family) every time I'm doing my BWRAF gear check when we get to the dive site, and as I pump some air into my wing as I stand up to get ready to step off.