A: Well... it's not as cool as it seems... "officially" it stands for "Sistema d'Animació Físicament Realista". As you may have noticed, that's not English but Catalan, my mother's & father's language. In English it would mean something like "Physically Realistic Animation System".
Q: And why "2" ?
A: Because once upon a time there was Saphyre... and this is a complete redesign and reimplementation of that primitive project.
As there are *LOTS* of other projects with the same name but different target around (try to google it and count...) it can also be some kind of bad joke if you pronounce it "Saphyre Too" :)
Q: And in what kind of applications is it supposed to be used?
A: In any application that needs to perform physically based animation interactively/in realtime... The main target are videogames, but it could be used for mechanical prototyping apps, for virtual reality simulators or for educational physics demos, so the choice is yours...
Q: Does it work?
A: At this moment, not really... A preliminary "kernel" and a simple implementation of each subsystem is available, but they are not ready to be used in a full project yet. There is some basic functionality missing and some stability issues that should be solved before anybody but me can use Saphyre without blood and tears staining the keyboard... so stay tuned and wait for the Beta-day.-
Q: Why another "realtime physics engine"???
A: While an oriental philosophist would answer to this question with another question like "Well... why another Apocalyptic-Future-FPS? Why another Sword-And-Sorcery-RPG? Why another beer?"... My answer is... "just for fun".
There are some other free realtime physics engines around in the web, with ODE (http://www.q12.org) being the most widely used and best supported of them. That engine has been coded by an ex-Mathengine guy and currently is the wisest choice if you want a fast & reasonably-stable rigid body physics engine. So go for it and "come back later" :)