Joseph Stefano at E! Online : "Actually, it never touches the skin...And the whole scene goes so fast, you think you see more than you do."
Janet Leigh at The Astounding B Monster : "People swear they saw the knife go in, but they never showed that -- it was not allowed."
Alfred Hitchcock, in an interview with Francois Truffaut: "Naturally, the knife never touched the body; it was all done in the montage." - Hitchcock (Revised Edition), Helen G. Scott and Francois Truffaut
The 10-frame sequence below includes the gut-wrenching frames in question.