First, flatten your hands into the standard "Karate chop" shape. Curl your arms at the elbows so that your fingers are now pointed forward. For a more visceral experience, set a piece of fruit -- a banana, perhaps -- on a surface in front of you. Use a cutting board if it helps complete the illusion. Alternately, if you're in the position to give a friend or significant other a sensual massage, have them lie down in front of you.
Now, when you (and your partner, if applicable) are ready, start chopping your hands up and down like a maniac.
Thankfully, it doesn't really matter, as developer Halfbrick has done its best to compensate for Kinect's shortcomings. We tried out the arcade mode, in which there is no penalty for missing fruits, and bombs don't end the game, instead deducting points if struck. There are also special fruits that, when sliced, trigger special events. A frozen banana, for example, will slow the action to a crawl, making precision slicing easier. Another special banana starts a "fruit frenzy," during which an enormous number of fruits fill the screen, many more than seen in the mobile version's Arcade mode. The game also supports two simultaneous players, making fruit frenzy particularly hectic.
Fruit Ninja Kinect essentially boils down to a round of absolutely manic -- and surprisingly entertaining -- flailing. And let's face it, Fruit Ninja was never the deepest experience to begin with. It has always been an enjoyable and brief diversion, the perfect thing to kill some time (and fruit). Fruit Ninja Kinect is much the same, playing to the strengths of its new platform. Of course, you can't exactly whip out a Kinect while waiting in line at the checkout counter, and booting up an Xbox is a bigger time investment than pulling a phone out of your pocket.
At the right price, Fruit Ninja Kinect has the potential to be a fun party game, or at least a good Kinect tech demo for non-gaming friends. Beyond that, it's hard to imagine spending too much time with it.
Update: As a few commenters have pointed out, it appears that what was on display at Microsoft's event was the Arcade mode, which is also in the mobile versions of Fruit Ninja. We've clarified a few details. The final game will include Classic Mode, though it wasn't on display at Microsoft's event.