two scenarios, both on a computrainer:
2x20 @ LT W, ergometer (fixed load) mode, typical pattern: 1min @ 50-60RPM, 1min @ 70-80RPM, 1min @ 90-100RPM, repeat till done
2x20 variable load mode, typically just over LT W, rising to LT+20, very occasionally dropping to LT-20 for 5-10 seconds
these two workouts feel totally different. would the real 2x20 please stand up?
Ummm both sound like 2x20 to me just different points to the intervals.
Same way that 6x300 (swimming) can be done lots of different ways.
Decsend 1-3, 4-6.
Descend 1-3, 4-6 negative split each
etc.
interesting reading, thanks. not sure why i hadn’t come across that before.
unfortunately, my interpretation of it is precisely the opposite of yours. AC’s diagrams and comments seem to show that different kinds of races feature a different CPV vs. AEPV profile, even though in every case normalized power and cadence may be broadly comparable. wouldn’t that suggest that one should use a training pattern that matches the CPV vs AEPV profile most likely to apply to the type(s) of events one is training for?
just speaking qualitatively, i can say that fixed power mode on the CT for a 2x20 is massively easier than variable power mode. my own anecdotal explanation is that fixed power mode removes any “mental focus” requirements from the workout - whatever your legs are doing, they are generating the wattage, so you no longer have to concentrate. in variable power mode, it requires constant attention to ensure you are in the correct wattage zone, and the moment fatigue sets in, reductions do not just lead to lower cadence (as happens for me in fixed power mode), but to lower wattage.
result: i prefer fixed power workouts but they seem unrepresentative of the demands of actually racing at anything close to that power. but who knows. i was at about 280W in may, able to do both styles of workout. right now, the fixed power one is quite manageable, as it should be, but the variable power one is not. i guess that means i should just HTFU and do the variable power ones 
I’ve been using a different approach. I do the intervals in PC mode, matching speed against a pacer preset to the desired wattage, traveling over a flat windless course. My goal during each interval is at least to maintain a constant, or if possible slightly diminishing, distance behind the pacer. Depending on how my legs are feeling, I typically go back and forth between two different gears and cadences. I think this approach more closely approximates how I would pedal during a race than an ergometer-mode workout would, and it’s also more mentally stimulating.