What is a Qsys BFM?

That point is clear, I understand that. The question is, when I create a Qsys system and then choose options to create BFM and a testbench with the simulation models and open modelsim and run msim_setup.tcl, in all this how does the BFM know what value it is supposed to put onto the component? Or do I have to add the BFM manually in the Qsys itself and otherwise they will not be added into the simulation model? How do I tell the BFM what it is supposed to do? I thought that the BFM just does things on its own. It has been created and it just works when we choose the BFM option in Qsys before we generate the system.