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When the _formActionUrl parameter is set but no Url is given (i.e. ...&_formActionUrl =&embed=...) the top frame will be closed. It is as if you set Form Action to Close" in the Form Designer.
For example... the Url http://www.frevvo.com/....share Url with &_formActionUrl=http://www.google.com, will cause the form to display google.com when it's submitted, even if in the Form Designer you had set the Form Action to be "Display Message" or one of the other options.