TraceDialog[expr] initiates a dialog for every expression used in the evaluation of expr.
TraceDialog[expr, form] initiates a dialog only for expressions which match form.
TraceDialog[expr, s] initiates dialogs only for expressions whose evaluations use transformation rules associated with the symbol s.
See notes for Trace.
The expression to be evaluated when a dialog is called is given as Out[$Line] of the dialog, wrapped in HoldForm. The expression can be seen by asking for % when the dialog is first started.
Any value returned from the dialog is discarded.
TraceDialog[expr] returns the result of evaluating expr.
See The Mathematica Book: Section 2.5.10.