Same issues in Workout 6 like Workout 2 with the wheel spinning out after the free ride sprints going into the low power sections. The only time it worked right was if I was coming out of the sprint and I was on a hill.
Also the resistance on the free ride sections felt totally different section to section and I wasn’t able to produce big watts twice.
I even had to unclip and use my toe to slow down my trainer twice (once almost cutting through my damn shoe).
setup: BMC Team Machine on a Wahoo Kickr Core with Apple TV
I have a kickr core too, and also failed the 30 sec low power sections following the free ride sprints. However I didn’t try to manually slow down the fly wheel, I just let it spin down naturally. It only takes 10 seconds or so to get some resistance back.
Workout 2 was fine for me other than what I described above.
I kind of wonder if the coaches who wrote these workouts ever tried them on a smart trainer, or have ever used Zwift to begin with. 
It was weird to me how it felt different interval to interval. I think there were 6 sprint intervals in Workout 6 and it felt like it worked as intended maybe once - where the resistance felt right coming out of ERG mode, where after the sprint the wheel slowed down normally and erg mode kicked back in smoothly and without issues
@Federico_Cribiore and @Mike_Rowe_PBR I don’t know whether this will help, but I had a similar issue with some of the initial FTP Builder workouts, and what I ended up doing was going into the sprint in a much higher gear and lower cadence. The training screen even said to go in at 40 rpm, but I was initially so freaked out at having to get to the higher wattage that I didn’t read that part and was somewhere between 90 and 100 rpm, no doubt. I haven’t done the Zwift Academy yet, so I’m not sure whether my experience is applicable here, but wanted to suggest - just in case! Ride On!
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Yep, just finished WO 6, failed most of the “sections” after the sprint because it takes the flywheel time to coast down / provide resistance.
Plus, all the normal ERG issues. i.e It only seemed to enable disable for the first 15 mins. Then it got stuck on disabled and I couldn’t turn it back on, then bluetooth disconnected and zwift couldn’t find my trainer, so I switched to the companion app, but ERG was still disabled/ not available. Luckily ERG became an option to turn back on after all the sprints. But, my power data for the ride seems to be a mess, despite the numbers I was seeing in Zwift. Strava is reporting my 20 second sprints as 82 watts 
Also so many “get ready to dig in” workout texts right before a 130 watt 3 min rest. Thanks zwift that wasn’t digging…
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I kind of wonder if the person who wrote the code that awards the stars for workout block has ever done a sprint interval workout.
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I’ve seen these comments before, but on ZA workout #2 had no issues myself on Tacx Neo (first gen). Haven’t done #6 yet