It's interesting that you say all Brazilian guitarists use some give in the tips. I believe I can hear the Assad brothers getting a tone that sounds like the tips are buckling, but I have yet to find a close=up to prove that. Can you post some videos of Brazilian guitarist's tip joints?
At least most of those I know and for sure all I have already asked this question said yes. I don't know the Assads, but I'll try to find out, my teacher is a close friend of them as well as Abreu.
At youtube there is a video of them playing Gershwin in black and white (search Youtube for "Assad in blue" and go for those under user "guitar magazine"). First video, part I, several close ups of Odair's RH. Resolution of the video is terrible. Around 0:10 nice close up which seems there is no collapsing. Minute 1:46 middle finger clearly collapses. A lot of non-collapsing strokes as well as some small almost unnoticeable collapsing (IMO). I would say again the school here is to vary tonus (flexor tension) from complete relaxation and collapsing to non-collapsing with a lot of finger flexion, and all variations in between those extremes, according to recquired quality of tone and dynamics (at least this is what I've learnt).
I would think like this, in a fixed hand position, knuckle generates main motion, if you tense tip more you move string more up, with collapsing string goes more down toward soundhole. you may also vary passing angle of nail with this. So you vary tone and sound result with those variation. In a cheap factory made guitar the variation very subtle, maybe in a Smallman too (just kidding
), but in a good concert guitar, it makes a good difference in sound result. Good enough to detach voices and enhance dynamic options.
Search also FAbio Zanon videos. There are some new ones with high resolution, search youtube for "Zanon Aire Indio", for example, and see the way he varies the collapsing. Sometimes more, sometimes less, a lot of varying angles, slipping strokes, hand position etc. He is one of the most beautiful tones and tone and dynamic variation I know.