Danted7 - yes exactly! For those who don't know what he's looking at - look here - http://milano.arounder.com/PROJECTS/SANTA_MARIA_DELLE_GRAZIE/
that is a very high resolution image of the painting. When you look at Peter's hand, it literally looks like it is coming in at a 90 degree (right) angle to the knife. Yes it is *conceivable* that a hand could hold a knife that way, but it is poor anatomy for a painting. Leonardo was a master of anatomy. It really looks like Peter is instead holding someone's hand that is trying to wield a knife and that Peter is *holding back* that hand.
Again this is the difference between first glance meaning and more intense evaluation. The first glance says "Peter holds a knife, just like the scriptures say". The more intense evaluation says "wait a minute, Peter is actually preventing an attack ..."