probably you should download the zipped solution at the end of the post and run it to see the difference.
Changes in TextBox are being throttled so that we don’t trigger the underlying databound property on each key stroke, but only after user stopped typing.

You usually want to throttle events if the handler code is CPU expensive and/or lenghty so it does not make sense to invoke it each time value changes, but only when we are sure that the value stopped changing.

Hope this clarifies it.
Thanks for your feedback!

I’m not sure I quite understand the point of this workaround. Why not simply set the UpdateSourceTrigger of the particular binding to ‘PropertyChanged‘? This way, the ‘SearchCriteriaText’ gets updated every time the ‘Text’ property on the TextBox changes.