The drill bit is 4.5 inches long, which doesn't sound too bad, until you actually watch it go down. According to always reliable internet comments, 4.5 inches deep is 3/16ths of an inch from one's brain stem. That sounds about right, but it's truly horrifying either way.
Of course, the "blockhead" bit is a sideshow classic, with a rich cultural history dating back to the earliest of circus freaks. Not that the historical context makes this any easier to watch.