The phrase "gnash their teeth" seems to be a Biblical reference. It's actually in a few places in Matthew where Jesus explains some parables, but Matthew 13:41-42 is probably the most easily quotable: "The Son of Man will send his angels, and they will gather out of his kingdom all causes of sin and all law-breakers, and throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth."
The strumming pattern emphasizes "claw" and "gnash" in the line "Claw at my skin and gnash their teeth and shout." Both of these words occur on the first beat of the bar and coincide with the most forceful strum.
