See, here's the receiver of that magic block.  The block defined in the call is passed as "&file_operation".  That variable points to an object, a procedure that can be called with ".call".  And just like every other method, you can pass values to it.

This method is responsible for the details of opening the temporary file.  It then passes the file to the block, allowing the caller to say what he or she wants to be done with that file.