Shotgunning liquids has been around for a while. It's the crude process of poking a hole in the bottom of a can (usually beer) so that you can drink whatever is inside the can way faster than normal.
It's a fun thing to do at a party for sure, but it gets old pretty quick, and there's never anything good lying around to make the hole in the can with. Using your finger to poke the hole raises things to a super-human level and is an unparalleled party trick for anyone who doesn't know that it's possible.
*****Shotgunning beers with your finger carries an inherent risk of injury and it's possible that your thumb could get cut as it pierces the wall of the can. While this has never happened to me, or anyone else I've seen try this, my brain tells me that it's certainly something to be aware of. Pierce responsibly and understand the risks involved of poking holes in metal cans with nothing more then your finger.*****
Step 1: Setup
Grab any normal aluminum can and position your dominant thumb (I'm a lefty so I use my left thumb) around 1" to 1.5" from the bottom. The blue line at the bottom of the Pabst Blue Ribbon logo seems to be about the right spot. Have the mouth of the can that you normally would drink from facing up towards the sky (at 12 o'clock on the top of the can).
Then, hold the can either in your other hand, or use your index and middle fingers to support it along the bottom of the can on the other side so you've got something to push against. See the photos below.
Think about squeezing the can between your fingers and visualize squashing it like a grape and practice rocking the tip of your thumb so that your finger nail makes a diving motion in towards the wall of the can.
Finger Nail Length
While your length of your fingernail is important for a successful finger shotgun, I don't think that it means everything. I've seen people with very short nails have success, and people with long nails have them double over and crease. The ideal length is likely to have around 1/8th of an inch or so of the white colored fingernail extending beyond the cuticle. That should be enough so that it can make direct contact with the can, but not so long that the nail becomes weak and flexible.