The default minimizer uses a sophisticated conjugate gradient and line search algorithm with much better performance than the older velocity quenching method. The method of conjugate gradients is used to select successive search directions (starting with the initial gradient) which eliminate repeated minimization along the same directions. Along each direction, a minimum is first bracketed (rigorously bounded) and then converged upon by either a golden section search, or, when possible, a quadratically convergent method using gradient information.
For most systems, it just works.
You can perform energy minimization using a simple quenching scheme. While this algorithm is not the most rapidly convergent, it is sufficient for most applications. There are only two parameters for minimization: one to activate minimization and another to specify the maximum movement of any atom.