Since it is a commercial engine the implementation details are unknown ... Most physics engines provide results similar to the Symplectic Euler integrator, or 2nd order Euler. Novodex (Ageia PhysX) provided the best results.
The integration error graph shows it behaving slightly better than the "Euler 2nd order" reference implementation, and significantly better than the "symplectic Euler" reference integrator. So it looks to me like it's not a 100% off-the-shelf integrator, and has some custom adjustments that may be hard to duplicate without the source code.