In movies about courtroom dramas, you will commonly hear objections from the prosecution or the defence to the judge about leading the witness. That is, the witness has to think and speak independently without live coaching or prompting. The panel-based defences for VCDX, CCAr and NPX have the same premise.
DISCLAIMER: This article is completely based upon my experiences as an NPX Certified Examiner. However, my panel training was by former VCDX panelists and moderators.