Can't understand where to know what physijs functions do. There are many functions like applyImpulse, applyForce etc. but there is no documentation for them.
Physi.js is a wrapper for ammo.js which is a port of bullet. I dont know of any good guides/tutorials for physi.js or ammo.js that covers more than a basic scene. However when you want to learn about what the functions are doing, you can look it up in the bullet physics docs.
What helped me understand were some bullet tutorials here: http://bulletphysics.org/mediawiki-1.5.8/index.php/Tutorial_Articles
and the Forum where you can find further information in some user posts: http://www.bulletphysics.org/Bullet/phpBB3/index.php?sid=0e8c6e5111583be0d17f1731e4c1dc19