So far I found the following.
Doing ‘Singing C’ C#:
Step 1 = OK
Step 2 = OK
Step 3 – NO way
Step 4 = OK
That is – I can’t do C#-F# slurs at ‘FF’ or ‘F’. To do that I have to engage too much muscular tension of the ‘Uuuh’ shape and trying to engage it I immediately tense up and Passive Release turns into Active Release.
On the contrary, when doing the same slur after C# has been reduced to ‘p’ involves no tension in the passive release.