For quite some time I did not understand what the Update Form button action was for or what it did, but once I tried it out, I found it to provide some pretty cool functionality for InfoPath browser forms.
You can set the Update Form button action by creating a browser-compatible InfoPath form template, adding a button control to it, and then going to the Properties for the button control. The Update Form button action is located in the Action drop-down list box on the Button Properties dialog box.
Office Online gives the following description of the Update Form button action:
In a form that is designed to be filled out in a Web browser, this action updates form data in an incremental manner.
I used the Update Form button action in an article I wrote about updating SharePoint list items via an InfoPath browser form using the SharePoint Object Model.
In this article, I used the Changed event of fields within a Repeating Table that was bound to a secondary data source of a SharePoint list to be able to update the SharePoint list items.
The fields in the Repeating Table were set to send data to the server only when necessary for correct rendering of the form. This is the default postback settings on the Browser forms tab for sending changes made to data in fields back to the server.
I used an Update Form action on a button control to be able to make changes to data in the fields of the Repeating Table and have InfoPath remember all of those changes I made in several rows (without posting data back to the server).
When I clicked the button that had the Update Form action set to it, InfoPath sent all of the changes I had made back to the server as one bulk update! Now that’s pretty neat if you want to improve the performance of a browser form. 
If you are looking for other ways to improve performance of your InfoPath web-based forms, you can check out the blog of the InfoPath Team, who wrote an article series dealing with performance in browser forms.
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