Battlefield 3 has some pretty realistic physics, but let's not get carried away here. It's a video game. Meaning those physcis exist solely so people can do cool shit like flip a tank onto the roof of a building.
I don't want to know how much planning and trial-and-error went into this, but I don't really care. The end results are worth whatever it took.
BF3 Stunts: Tanks are Flip'N Awesome! [YouTube]