A one handed break that I made-up when I was in Junior High School. It could be the "official" way to do it, or it could be really poor form. I wanted to do a one handed cut, and there was no Internet to search back then, so I had to figure it out for myself.
Turns out it's a slight variation on one posted by FrenchCrawler about a year ago, so I must not have been very far off the mark.
Step 1: Hold the Cards
Take all the cards in one hand. Hold them with your four fingers (assuming you have four) along the back of the deck, and your thumb on the other side. You want to just bearly get the tip of your fingers over the top of the deck.
Step 2: Drop the Cards
Roll your thumb up so the bottom of your thumb comes off the cards. This should let part of the deck drop into the palm of your hand.
Step 3: Push Up the Cards
Use your index finger (that's the one next to your thumb) to push the back of the cards up, piviting on the front of the cards at the base of your thumb.
Step 4: Pinky Work
Put the tip your pinky on the bottom of the deck still between your thumb and your fingers. You'll use this in the next step.
Step 5: Pull Back!
Use your pinky to pull the cards back, and at the same time, use your index finger to pul lthe cards forward.
Your pinky, at this point, is also keeping the bottom card from sliding forward.
Step 6: Drop the Cards Again
Drop the top cards into the palm of your hand.
Step 7: Close the Deck
Use your thumb do close the deck.
Step 8: Videos
Here's a not so clean cut.
And here's a really not clean cut.