For the sake of debate.....Look at page 32 of the July 2007 issue of Boxing Digest. Note the position of the fist as Marquez lands that hook. I think that, if you take a survey over the years of photos of left hook landing, the huge majority land with the thumb up. BUT....I think this is the source of the confusion. You start the punch with the thumb up, but end the left hook with the palm down and the knuckles touching your right shoulder, so it turns over en route...I don't think you are best served by landing it with the palm down.