StackComplete[expr] evaluates expr with intermediate expressions in evaluation chains included on the stack.
Mathematica normally includes only the latest expression on each evaluation chain involved in the evaluation of a particular expression. Inside StackComplete, however, all preceding expressions on the evaluation chains are included.
StackComplete typically increases significantly the number of expressions kept on the evaluation stack.
See The Mathematica Book: Section 2.5.11.
See also: TraceBackward, TraceAbove.