there’s two ways to do this and I’ve used both.
(1) you can turn-off your KICKR as a controllable trainer from the (A) pairing screen. then after starting a ride with Zwift, open the Wahoo Fitness App on your iPhone/iPad, make sure it’s connected to your KICKR, then start a workout in the Wahoo App. now you can use any of the resistance modes (e.g. Level, %, ERG, Sim) in the App to control your KICKR.
doing it this way means you totally control the KICKR and Zwift will never control the trainer.
(2) let Zwift control your KICKR as a controllable trainer but then use the Wahoo Fitness App to override resistance. start up Zwift and start riding, then open the Wahoo Fitness App, connect to your KICKR, and then start a workout in the Wahoo App. When you go to the Sim mode screen, you’ll see the Slope % change in real-time as Zwift controls the trainer based on the gradient in-game.
You can +/- Wind Speed and this will affect the resistance significantly while on the flats. Don’t try to use Wind Speed to affect resistance on hills, it won’t have the desired effect. I’ve generally used wind speeds of 10-20 mph to add/remove resistance. If you go too high (e.g. 50 mph) things start to get screwy and the KICKR seems to get confused. I’ve also had the Wahoo App crash while experimenting with high wind speeds.
You can also switch to any of the other modes, such as ERG or Level and then control the KICKR. _ I think the trick is to switch to one of these modes and then change the resistance quickly before Zwift sends a command to the KICKR which will kick it back to Sim mode. _ Once you’ve successfully taken over the resistance, Zwift shouldn’t be able to force the KICKR back to Sim mode. If Zwift does force the KICKR back to Sim mode, just try again until it works. When you are ready to let Zwift take over again, just go back to Sim mode and Zwift should start to take over and control the resistance. I’ve successfully gone back and forth this way many times during a ride.