I renamed some variables to make the source a bit more clear.
Basically the JacobianEntry calculates and stores some values to perform constraint correction:
for two rigidbodies, A and B it precomputes
angular Jacobian entries J for both bodies
Minv Jt for both bodies
Adiag, the diagonal entry in the system matrix A = J Minv Jt
This information can be used in a Gauss-Siedel iterative LCP solver, or to correct the constraint error on a per-body pair.
Once I got some time I will add some more details, see the renamed variables in JacobianEntry here:
http://www.erwincoumans.com/Bullet/Bull ... tated.html