Very often, once I said I practice karate, people will show me the 'Chop chop' sign. This is the misconception of karate-do. I have been practicing this form of martial art for more than forty years, but I hardly use this 'Chop chop' movement on subject nor person. Reason is very simple, I take up the art to build my physical and mental 'Me'; not to use the art to hurt people or to 'Show off'.
But I do see lots of people using this technique on stage and reinforce the concept of 'Karate'.
Are the chopping movement really using the blade or edge of the palm while executing? I found this slow motion clip on YouTube. It may provide you the answer. Look extremely careful at the moment of the 'hand knife' contacting the slab. You should see how the edge of the palm turns and part of the wrist follows in to add the force to the slab. Amazing right? So is this a Hand Knife Technique?