HoldAllComplete is an attribute which specifies that all arguments to a function are not to be modified or looked at in any way in the process of evaluation.
By setting the attribute HoldAllComplete you can effectively shield the arguments of a function from all aspects of the standard Mathematica evaluation process.
HoldAllComplete not only prevents arguments from being evaluated, but also prevents Sequence objects from being flattened, Unevaluated wrappers from being stripped, and upvalues associated with arguments from being used.
Evaluate cannot be used to override HoldAllComplete.
See The Mathematica Book: Section 2.5.3 and Section 2.5.5.
See also: HoldComplete, HoldAll, SequenceHold, Extract.