The idea is to spend the most time operating at or near VO2max to force the body to adapt, but the training effect isn't really down to time in power zone but rather physiologically getting you in the state near maximum oxygen consumption and keep you there as long as possible. There's more than one way to get there and you don't quite need to test and retest FTP frequently or ERG mode to do them correctly.
What's more indicative is getting the heart rate response to closely replicate what happens when you're doing more traditional longer intervals; if my block of 40/20s is giving me the same heart rate rise as doing longer VO2max intervals on the road does, the intensity of those 40 second work periods is pretty much right. For me (and looking at studies I'm not unique in that regard) the 40/20s interval format works better than the 30/30s where I'd have to really ratchet the power up more to get the same response.
Last edited by Branko D; 02-12-21 at 04:10 AM.