This looks a little like that scene from The Hangover, except the big guy who doesn't go down isn't Zach Galifianakis but Missoula Police Chief Mark Muir. Chief Muir takes the shot during the Citizens' Law Enforcement Academy, and explains that he's never been tased before. Rookies go through taser training, so Muir figures he's due.
There are two videos, from two different angles. The first starts right at the shot and shows Muir's reaction.
The second includes Muir's explanation for volunteering, and shows the shot from behind.