What is ERG Mode on my Trainer?
Nails on a chalkboard...
ERG mode on Smart Trainers is a setting that fixes your power output based upon the workout provided by your coach by automatically adjusting your resistance to match your cadence in order to maintain the designated wattage goal. So, as power is a function of how hard you are pushing on your pedals multiplied by cadence, to maintain a particular wattage, you either increase your cadence or increase torque and push your pedals harder.
The problem with ERG mode is that as you train, you get stronger, so your current FTP is always a lagging indicator of your true capability so sometimes the workout is too easy; or, if you are having a bad day (e.g., sick, tired, unmotivated), you can't hit the numbers and quit as it is too hard.
Therefore, don't use it!
What I recommend is to use the "Lap Function" on your bike computer or watch to track your wattage for a given interval so that, if you are killing it that day, you take advantage of your capability or, if it is not your day, you curtail your effort accordingly and work to follow the prescribed plan, but do so with wattage you can maintain.