enhancement methods

Dear Experts,

Iam hust going through a SAP Book .in that it is written that there are in two ways we can do enhancement.

1. by Enhancement(Append Structure)

2. by modification

create a view or an ABAP query infoset and overwrite the extractor for BW and the extract structure in the maintainance function for data source (MD Attributes)

can any one explian me about the second method with screen shots for better understanding.....

thanks in advance

Regards

Sabiha