In fact, if I was sure I'm not doing the exercices a wrong way and so I will be able to strenghten my upper range within several months, I didn't think so much about this break position and simply did the exercice without thinking. I don't know if I will be able to post a clip, but the thing is, doing RYV transcending tone exercice, in the C4 to A4 or sometimes B4 range I clearly feel in my vocal folds a switch from falsetto to a full voice "strong" mechanism, but above A4 or B4 I don't feel that switch anymore and my voice tends to me lighter, seems like my vocal folds don't have the ability to do anything else but falsetto.
The same thing occurs with the Siren Exercice, i'm able to do the siren in full voice from start to end in the C4 to A4 range, but above A4 my voice flips into a lighter mechanism.

Same thing - perfectly normal. Actually, you CAN keep that incredibly thick sound in a siren all the way up to C5, but most singers makes some switch before that (usually somewhere between E4 and A4), even very powerful ones.

thank you for the answer
So if it is normal, di you think all I have to do is to carry on practicing the exercices, hoping that with practice my light mechanism above the A4/B4 break will strenghten so the gap will be less obvious (in RYV illustration, maybe currently above A4/B4 I sing with a mixed voice close to falsetto and with practice this mixed voice will come closer to full voice???)

Your break can be different then what described in the book. Don't worry about that. To be able to switch gears properly, you must create the right vocal path for the voice (and resonance) to travel in the mask by tilting the larynx so that it can pull the vocal cords properly. This is done by dropping the jaw correctly as in the yawn sensation. You also need the right amount of air pressure sent by your diaphragm.

It is difficult to say what exactly you are doing that is coming in the way of successfully completing this exercise. If you need some guidance, I'd be happy to help you in a one-on-one lesson to get you started. Here's my profile:

http://www.rocksource360.com/live-online-vocal-instuction/valerie-bastien.html
Good luck!

Val

Working on RYV main 3 exercices + Reinforced Falsetto, I'm now able to smoothen a little bit that "break" and my upper range begins to strenghten a little bit, but now I experience an new problem. 2 weeks ago I was able to reach G5/G#5 in falsetto, now I can only reach a F5. Are there some exercice to counter-balance that?

May be you're not as relaxed? Let the voice be the guide and go where it wants to resonate! I will come back again!

that's weird, I don't think I'm not as relaxed as 2 weeks ago. Maybe am I more tired, I don't know.