Yes, this is most definitely possible, but requires some simple coding.
Create an onAfterChange event handler on the field where a user can type
"yes". In the handler, check the value of that field (make sure it's "yes")
and insert an optional section (using XDocument.View.ExecuteAction()). Good
luck.
> I'm new to Infopath, trying to create a form, I want to automatically
> insert
> optional section when user response to yes in a field rather than choosing
> to
> insert. Is it possible? Thanks for helping.