LarryE, KitaYama doesn't want the main form to save when doing this selection. Selecting at least one record in the sub-form will do the unwanted action and that is the nexus of this discussion.
If you have no user-defined BeforeUpdate event, it doesn't matter - because Access DOES have its own internalized BeforeUpdate event. What did you think actually fired YOUR BeforeUpdate event? An update will occur whether or not you have any VBA in place. Just like a form will have an Open, Load, and Current event whether you added your own macro or VBA. In fact, it is possible to have a basic form with sub-forms that have no code ANYWHERE. Every event that could happen to such forms WILL happen.
Now, if you dirty the main form then switch to the sub-form, the main form data gets saved during the sequence of events triggered by the change of focus. If you then switch to a SECOND sub-form on the same main without stepping into the main, it won't matter, because the main form is no longer dirty and you don't save clean forms. BUT having no BeforeUpdate event wouldn't prevent the initial main form data save.
It is sort of like the old conundrum of "If a tree falls in a forest and no one is around to hear it, does it make a noise?" Well in this case, "If a bound form changes records but has no VBA code or macro associated defined within it, does the Current Event still happen?" In the latter case, YES. It doesn't do much - but it still happens.