JetPens explains it well:
The pen writes in extreme conditions including wet surfaces, cool temperatures, and even upside down. Normal pens utilize gravity to create ink flow onto a paper. But the Uni-Ball Power Tank utilizes internal compressed air pressure to push out ink onto the paper. Revolving air inside allows it to continually push ink out until the end. The pen can write in weightless conditions such as in space, on wet paper and subzero environments. The oil based pigment ink is high density and comes out smooth and dark on paper.
To test its ability to write upside down, I tried writing upside down. That was a failure. So instead, I wrote against a wall for a few minutes. The Power Tank kept writing as though I writing like normal on a flat surface! If I were writing with a regular pen against a wall, the ink would stop flowing. I'm sure this has happened to all of us before.
Notebook Loves Pen
Pretense of Knowledge
The Pen Addict
JetPens - $3.30 (refills are $1.65)
If you have any interest in ballpoint pens, or you are looking for one or two special ballpoints to keep in your pencil case, I think you should consider the Uni-ball Power Tank! It's cool and rugged looking, and writes very well. It is not a ballpoint you'll want to lend out though. The Uni-ball Power Tank comes in a variety of models, colors, and tip sizes if this particular model doesn't float your boat. My only complaint is the blue ink is not dark enough.