So I’m new to this and enjoying starting out on the training programmes.
The one I’m doing asks for a certain watt / cadence combination for a particular duration.
On the screen it shows “spin faster / spin slower” and/or “more power / less power”, which is helpful to try to get you into the right speed.
As a beginner It took me a bit of time to figure the right gear to be in. Given that with a fixed resistance, in a particular gear cadence and speed (power) are directly related, it should be possible to also say “gear up? gear down?”.
In fact a test of going through the gears at a particular cadence probably would give zwift enough info to tell you what gear you ought to be in…
Probably not going to happen as many people do workouts in ERG mode on a smart trainer. In this mode there’s no need to change gear as the trainer adjusts resistance to force you to ride at the target power level. You then ride at whatever cadence you desire or at a cadence directed on screen.
If you’re on a dumb trainer then it’s up to you to change gear to match the target power and cadence. You soon get the hang of it.
Well I’ll be damned - my trainer supports this and I didn’t even realise!
Makes total sense. I think the first time I tried a session it didn’t use it - this is maybe because I was using a cadence sensor on the bike instead of using the trainer’s data for that.
It is a bit confusing though as it still says “reduce power” occasionally which isn’t really possible in Erg mode. Perhaps it should only say spin faster/slower 
Yeah mine will say reduce / increase power as well and there’s nothing you can do about it since I’m in ERG mode. Just ignore it and wait for the trainer to settle down on target power. This is mainly for the benefit of people on dumb trainers where they can either spin faster or slower or change gears to keep on target.
Or people on Smart trainers that does not use ERG mode.
Or people on smart trainers running them out of ERG mode as i do sometimes.