Not true– each row has its own complete set of fields/controls.
What is happening is that on a change of row, the subordinate tab (grid?) is hidden on the previous row and shown on the current row. "what event forces the movement of the lines n a subordinate grid In a form" There is no event exposed to scripting.
Working with detail areas is complex and working the detail areas that have subordinatetabs even more so.
For that reason, we have not thus far supported adding fields to detail areas viascripting. I know that does not help you much but it seems its very complex. I can think of adding a handler to check for lostkeyboard focus, and adding a handler for all events, including the handled events as well, working with the visual tree to locate the different areas but I think it is probably too complex to get good code quality in the script due to the complexity and the lack of debug support.