Is a PF An Automatic First Down?

[OKREF]

One the way home from a game last year, and listening to a radio guy say the following. "Block below the waist on the defense! That's impossible, the defense can't do that. These guys are horrible. I just don't understand where they get these guys"

Just so happens our crew was from the town he does radio for. We called him and told him that it was possible. Defensive back came up to try and play a sweep, and cut the offensive lineman at the knees.