That's a special case designed for the function object. It sets the flag to enable undefined expressions. If the user types an expression name in the object which is not recognised, the runtime calls ExtObject::ReturnUndefinedExpression with whatever parameters were passed to the expression - so there can be any number, and any type. The one after the last parameter in the params array passed to every action, condition and expression always has a type of EXPTYPE_NULL, therefore the parameters can be dynamically counted and retrieved.
I don't think this is supported by ordinary expressions I'm afraid.