Warning! Dumb questions about smart trainers coming up...
Never a dumb question!
I think I understand this bit now - when in ERG mode, if the SF app calls for a power of 250w, then a smart trainer will hold 250w watts no matter if you're doing 60 or 90rpm. If you're not having a good day and can't hold the power, you're stuck in a downward spiral?
Pretty much, although bear in mind the power curve of the trainer comes into things a huge amount in intervals, less so in more gradual gradient changes like climbs. Many/most trainers 'struggle' with short intervals on ERG - the power curve being too slow for large leaps in resistance, while still not feeling like a 'wall' of impossible resitance hitting your legs. In theory the power curve is there to 'hit' you just as your legs are pushing that wattage, but in reality you'll always be trying to 'catch' the resistance and power figure due to the curve, trying to stay on top of the gear. Ideally you need to be kicking just as it starts to ramp, rather than reacting to the ramp, much like you do on the road if you know the climb/kicker. You also need to be somehow sustaining the cadence and not changing gear as both of those will make it harder and longer for the trainer to 'hit' the required target, or more accurately to have supplied enough resitance for you to have hit the power target at a constant cadence.
IIRC, both DCR and GPL commented the new Saris H3 is excellent for hitting the target power for intervals, so a great choice of TrainerRoad (and thus I presume SF). Though I think I picked up that very short intervals (5 to 10s) aren't good in ERG mode as the trainer can't respond up/down quick enough - right?
See above on my take of why. I've heard that about the H2 and H3 from DCR but no experience. I think it probably just has a quicker/steeper power curve, or is able to hit it's measured power quicker/easier by a more progressive but stable power curve/ramp so you can hit interval targets with it quicker and more consistently.
If you're just riding along in Zwift, what mode do you put the trainer into? I presume you use gears like on the road?
By just 'riding along' you'll want it in ERG - you want it to be responding to the terrain you're riding. If that's what you're trying to do (which is one of the beauties of trainers, they can do lots of different things regarding what you're trying to do...)
Some of the reviews for Elite Suito note it doesn't have a power meter built-in. So what's the weakness of this?
Less accurate power measurement - it's taken from the 'speed' of belts/resistance units/flywheels and calculated into a power output, rather than a strain gauge like a power meter. They may be less accurate (as they're more of a reaction to your power) than a direct measurement of the actual power (which from a trainer with a power meter must be at the resistance unit).
Disclaimer: above is my understanding of it all, some of it possibly inaccurate.
Sometimes like last night I'll use manual gearing to affect power, along with slope emulation. 80% slope emulation works well for the Direto in Zwift, in as much as it can emulate up to 14% gradient and the max slope in Zwift outside the UCI Horrowgate course is ~18%, which means you will feel pretty much all slope changes... But for most climbs, setting slope emulation to 100% means your gearing and power output will match outdoors more closely, without factors like nasty headwinds etc.
Great point. I used to ride at the default, which I think then was 50% emulation. I found back then I could easily overpower the on-wheel trainer, there wasn't many big gradients on Zwift then, so even the steepest (maybe 12%?) only 'felt' like 6%. Didn't even need to stand, just ramp up seated power. When racing though, I found myself easily dropped by other riders on ramps, when I knew I had the power, I just couldn't lay it down. I upped my slider to 100% and it was spot on, really tough but I was still able to overpower it slow power/resistance curve of the on-wheel trainer. Switching to Direct Drive I've actually dropped it down to about 75-80%. It's meaty enough that I get the feel of everything, I can't overpower the trainer like before, but equally I tried to ride AdZ with it and I think the KOM climb at 100% and it was too much, 14% felt like 20%+ outside. A standing low cadence grind. It's about right now to how I ride outside and how climbs/ramps feel to me outside and more accurately to the power output and cadence I see from outside rides at similar cadence/efforts. Horses for courses really, if I wanted to do more power training and using free riding hills/AdZ repeats for it I'd probably put it back up to 100%.
Since Jr came along we've had so many of them. I know they're not 'environmentally friendly' but screw that - I've never really used them before so still amazed by their cleaning properties lol! 
