You're half right. Velocity can indeed be sensed from the pressure or flow. But it affects the note's volume only when it's first struck (attacked, turned on). If you're squeezing hard when you press a button, that note will play loud -- and continue playing loud, even tho you ease up on the bellows. Likewise if you hit it soft, it will remain soft no matter how much harder you squeeze, until you release the button and hit it again.
Actually, upon looking at Jordan's 'site again, the info on the MBE says that it has a: "bi-directional differential air pressure sensor, sensing the pressure value and bellows movement direction".
He says that it also has a: "selectable air-pressure MIDI parameter (Expression / Channel Pressure / Breath / Velocity).
I'm not sure how he'd set "Velocity" as a MIDI parameter using the pressure sensor. The others are CC's (continous controllers). Velocity is usually sensed by the time it takes 2 contacts under the key to close in succession. The the shorter the time between the closing of the 2 contacts, the higher the "Velocity" that is sensed.
So no swelling and fading of notes, which is pretty non-concertina
I had a cheap little Yamaha MIDI saxophone that worked that way, and it was so unreal I sold it after a week.
MIDI Volume is a better way to go. "Expression" may be even better, depending on what your synthesizer box can be programmed to do with it.
Yes, I want to build one of these too. And I have a friend who's built some of Jordan's boardds into his electronic organ, and they work fine. However, Jordan's MIDI Boutique is in Romania or Bulgaria, and getting payment to him can be tricky.