The Double Flip gesture recognizer uses both the 3-axis accelerometer and the 3-axis magnetometer in the phone.
There are many factors affecting if a Double Flip can be successfully triggered.
But the simple best practice to trigger a Double Flip is to:
keep the phone more horizontal and less vertical,
keep the phone pointing away from the magnetic poles,
and rotate along the phone’s long axis smoothly (not too fast).
If you are interested in technical details on why it works so and when it doesn’t work,
you can read the detailed description here. So far we have only tested the feature on a limited
set of devices including Nexus One, Motorola Droid and T-Mobile G1.
The sensor configuration on other devices might vary.