Chris Gossard nailed it! Take a look at the screenshot below, where I show the numbers of the first half of the interval done in ERG mode and the second half done in Sim mode.
You can see Chris had excellent control, and his power output for the controlled ERG mode section 1 vs. Sim mode section 2 was ONE WATT off. That being said, you can see that the Sim mode is more stochastic and jumpy.
So which is better? Well, both play a role in building performance. We will keep building now to turn off ERG mode more and more, but in a blended fashion, mixing ERG and Sim.
Now look at Chris's heart rate. By adding surges to the Tempo effort, we increased the demand of the effort, which, particularly over time, increases strain. If you watch the recording of our Advanced Training Principles webinar, you'll see me discuss this and demonstrate an additional aspect of adding surges (basically simple stress, more strain late in workout).
Great work, Chris!