Hey, @All-weather-danny, I thought I’d come back to your thread in more detail after having posted quickly late last night.
As Tante_Lotte alludes to, it’s probably thanks to the power reading in Zwift and our otherwise static position on the trainer that we can observe this phenomenon so easily.
I’m an inveterate tinkerer with my position on the bike. When I first got back into cycling through Zwift about three years ago after a decade or more off, I didn’t go in for a professional bike fit, but decided to DIY it on the trainer, benefiting from having the tools and the time.
Over these years, I’ve changed my position quite radically as my cycling ability has progressed and my core body strength has increased. When I first started experimenting with stem length/height and saddle position, I gravitated to an undemanding position with a shorter stem and the saddle quite forward. (A professional bike fitter would probably have steered me away from that immediately, but hey – no one got hurt.) This position was easier to hold for long periods, but, I found, also limited the power I was able to put out when on the hoods or in the drops. Like you, I noticed that when sitting up I was able to put out a lot more power. In addition, I found it particularly uncomfortable to transition from sitting up to the drops, feeling little control during the move. Like Tante_Lotte described above, there would be a brief but considerable peak in the power as I shifted position.
As my core strength has increased (and my weight has dropped) I have found that I’m now more comfortable putting my saddle much further back and having a longer stem. This stabilises my position on the bike overall, allowing me to go from the drops to sitting up and back again without huge spikes in power to the pedals, and feeling much more in control, while developing more power.
After a while, I realised that I should use the technology available and I started filming my self on the trainer side-on using my phone, while experimenting with position. Quite an eye-opener! That helped even more and I then went down the rabbit hole of changing crank length, experimenting with cleat positioning, etc. It’s lots of fun for me, but most people would probably prefer to get an experienced fitter’s opinion: far quicker, though likely even more expensive.
Everyone is at a different point in their cycling life. Perhaps you’re a far more experienced cyclist with a great position already and your issue comes from something else. However, examining bike fit and perhaps having someone else observe yours could provide a shortcut to a better overall riding experience for you, on and off the trainer.