My DS hasn't done a WISC yet, but I have this concern with the CogAT he just took at school as a screener for the gifted program. And in that test, you can't explain your answer to the tester.
They worked on some practice prep in class beforehand just to get them familiar with the types of questions and boy do I hope the actually questions were better written than the practice ones! I remember reading ds's answer to one fill in the blanks in the sentence question in which his answer made total sense to me, but was clearly not the intended response.