Can someone please clarify update behavior for me?

Given a form with fields from a row-buffered record in the parent table, containing a grid with records from a table-buffered child table: (Ex.: Invoice form, line items grid.)

When fields are filled in on the parent table record, and the cursor is moved to the child table grid, does a TABLEUPDATE() occur on the parent table record? (Because of the change in SELECTed table.)

I'm getting a significant number of parent records with no child table data entered, and my reading on the topic so far has been confusing.

Thank you,
Neil Preston