Rich Wharton had a tweat pointing at a Paul Smelders blog post here titled What EXACTLY is happening when you can’t keep up on the CT.

He gives some of the math involved and explains why you can or can't keep up when doing an ERG mode workout.

An ERG mode workout is where the CT attempts to keep you at a specific wattage. You cadence (speed of wheel effectively) is ignored. Just what your power output (watts) is. If you are putting out too much, the CT lowers its braking, not enough and it increases the braking so that you need to increase torque and therefore increase power.

This is great as long as you are in a Zone where you CAN keep up... Say the target is 220 watts... Go a little high and it eases you off. Go a little low and it pushes you back up... Net result is you do 220 watts +/- a very small amount for the prescribed time.

But if you are in a Zone where you CANNOT keep up... Say the target is 350 watts, now for the first while things go well. You hit the target and the CT keeps you there... But as you fatigue and can no longer stay at 350 watts, the CT notices and wants you to go up... So it increases the breaking so that you will increase torque.. Of course if that is more than you can do, more likely it will cause you to fall farther back as your cadence slows, and the CT will then increase breaking, and the end result is you stop completely....

This was timely... I missed getting out on the bike yesterday as I had to run out to Richmond to get the "race package" for the BC Senior Games... And it was a beautiful evening... Today I need to do a race tuneup ride for the ITT tomorrow and it is raining... So I may end up on the CT...