So do you mean, start on the principal note? e.g. I start on B, hold for a bit, then play C#, B, C#, B, C#, B as a trill, then repeat with the other notes, A, G, F#, E? Why is it necessary to hold the principal note before going into the trill?
It is not necessary to hold ! It's just a suggestion, a possibility.
Cortot and Eleanor Bailie suggest to start on the principal note.
Eleanor suggests to play B, C#, B, C#, B, C#, B continuous, and Cortot suggests an emphasis to the first note, B, in different a value note ...
Cortot sugests, B = 16th note.
and C#,B,C#, B...as 32th or 64th note, depends on number of notes of trill and your tempi.
The octaves at left hands will guide this ritenuto and crescendo passage.
I play with that emphasis they sugests, with a 16 th note to B, with a bit more emphasis, not a stop as I wrongly say.
I begin on principal note B as a 16th note... followed by 4 notes C#,B,C#,B (32th notes). This is my trill at my tempi. The same pattern to the following trill notes of the bar.
Elanor Bailey sugests only the emphasys at the first B, without different rhythms and Arthur Rubinstein plays that way.
I think that previous trills on previous bars, shoud have the same analogy and pattern to the chosen pattern above... as, same value notes on trill or not.
I hope my horrible english could help you any way.
Best Wishes
Pedro