In general, form in Trace[expr, form] is compared both with each complete expression that is evaluated, and with the tag associated with any transformation rule used in the evaluation.
Trace[expr, lhs->rhs] picks out expressions which match lhs, then replaces them with rhs in the list returned.
All expressions in the list returned by Trace are wrapped in HoldForm.
Trace returns a set of nested lists. Each individual list corresponds to a single evaluation chain, which contains the sequence of forms found for a particular expression. The list has sublists which give the histories of subsidiary evaluations.
whether to look at expressions before their heads and arguments are evaluated
During the execution of Trace, the settings for the form argument, and for the options TraceOn and TraceOff, can be modified by resetting the values of the global variables $TracePattern, $TraceOn and $TraceOff, respectively.