I have been doing endurance work since the weather turned in late fall. At that time, I changed to “ no decay”. Now, I’m a few weeks out from the start of a build phase and I want to go back to “optimal decay”. When I switched the toggle, however, my TP dropped by 45 watts. I assume that it is going all the way back to my last breakthrough signature and moving forward from there. How can I get the optimal decay to start with my current signature moving forward? Thanks.
I think you just have to lock your last workout, then the switch would start from there.
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How long ago was that?
You can always ride a BT workout from the library to validate your current signature values. That will establish a recent breakthrough event to use moving forward.
Even though you have been focusing on endurance for months you want to test your boundaries every so often. XATA will warn you when you signature is stale (over three weeks without any BT events).
Maybe 3 months. I find it nearly impossible to do indoor breakthroughs, especially since I can’t even approach my peak power from outdoors.
The comments in this thread are correct. Decay works by looking at your last BT (or locked signature) and your current TL to make changes to your signature. If you haven’t had a BT in several months, than it’s not surprising that your signature dropped substantially (intended behavior).
To adjust, simply go to your last ride before you want to make the switch to Optimal and go to Advanced MPA > Save. That will save & lock the signature, and then recalculate all data after that activity to the present.
Why type of trainer do you have?
Are you riding one of the BT workouts in slope mode on a periodic basis?
You don’t have to hit a PP high to get a BT, but you do need to sprint as hard and as long as you can – in a fatigued state – when you hit that portion of the workout. Points along your power curve are sampled in a similar way.
It’s the pattern of fatigue as MPA draws down that determines your signature values.
Reference: Breaking Through the Xert Way! – Xert
In the linked post under 2. it sounds a bit like it will only apply the new decay method to future workouts. Like the switch would take place at the time of changing the setting. In my experience it always goes back to the last BT or locked workout and recalculates from there. Same as Steve mentioned in his question.
So to get the result of changing at a user defined time - like changing between outdoor and indoor riding or between different training phases or whatever reason - one needs to lock a workout where the switch should take place (today or in the past) and toggle the setting to the new decay method and that affects the timeline from the locked workout into the future.
Is that the way it is supposed to work?
I have a Tacx Neo 2T.
I played around with it again and now I’m not so sure. It is a bit confusing 
The locking as a ‘stop here’ sign seems to work though.
And I remember that I set a signature by hand at some point and that might have screwed with the experiment.
I deleted some stuff that was off topic and I don’t really want to talk about me, sorry.
Correct. The switch is made immediately to your signature now and moving forward once you change decay methods but it doesn’t recalculate all signatures between the last BT and now. Hopefully that makes a little more sense?
Ahh, I think I get it, it leaves the old workouts as they are but calculates the signature from the last BT or locked workout till now and applies that to the next workout in the future?
Exactly! Perhaps I could improve the wording in the future… sorry for the confusion.
It’s a complex system. It’s what makes it so fun 
Well, I tried the Breakthrough #3 workout today and failed on the last, long 5 minute interval of the first set. Got caught in the low cadence spiral of death. I thought I had a decent sense of my TP, but now I’m not so sure. I guess I will let decay bring me down and attempt breakthroughs more often.
Any other advice?