I have questions about the principles of Ligandfit docking

Dear Sir,
I have questions as following about the principles of Ligandfit docking.

Why do the rigid body minimization in docking process?
What are different means and functions between using steepest descent (SD) algorithm and Broyden-Fletcher-Goldfarb-Shanno (BFGS) Quasi-Newton when do the rigid body minimization in docking process?
I just can not actually understand the correct docking steps in Ligandfit from help.
Could you give me more detail in Ligandfit docking? The more detail knowledge in Ligandfit docking is what we want to know.
Thank you very much. Looking forward to hearing from you soon.

Best Regards,

Mark