Need to change the BillToSite on detail lines if the Topmodelline is changed

It was suggested in a different thread to look at the

OE_Dependencies_Extn

package.

Sample line which is there is below. Looks like this will trigger a change in a dependant field if  the source field in the same line changes.. or the field on the header changes. I want the change the detail line based on billtosite on topmodelline. I see that happening for shiptosites..something comes up which says.postquery( if I enable customcode windows ) which makes this change. I want to track where that code is and add billtosite and 2 more fields in that to avoid invalid split invoices. Any help will be appreciated.