Billat had a study a few years ago where after establishing max aerobic uptake the power was decreased and max aerobic uptake was maintained for 15-25 minutes (depending on participant). In one case a participant dropped below ftp for a majority of the time IIRC.
I think vo2max work benefits from focusing on your breathing and making sure you maintain it. Some interesting stuff on the latest SpareCycles blog post (he is using vo2 mask):
Really just a great series of vo2max posts, in particular the most recent with actual aerobic uptake data. Like so many other things, training in the zone is a mix of art and science as your body isn’t a regimented machine that always behaves the same.