is a method of converting bytecode into native machine code, rather than Android’s Dalvik VM simply interpreting the bytecode. This speeds up apps and overall performance drastically. Cyanogen’s Nexus One build Beta 3 included JIT and has set the top speed on BenchmarkPi; OpenEclair 1.0.1 included JIT and absolutely flew.
Bill Buzbee, a member Google's Dalvik team states:
"To restate, the choice is between a near-immediate good performance boost vs. a better, but delayed, performance boost. An important use case for us is when a user downloads a new app from the market. Anecdotally, we think that for many users there is a very short window in which the first keep/uninstall decision is made – on the order of a few seconds. If the application feels sluggish on first use it may get discarded before it has a chance to shine. We didn’t want to require that an application
build up a usage profile over a series of runs, or introduce a big up-front compilation pause to benefit from the JIT.”
And now JIT is available for your Android running a 1.6 custom ROM's such as Cyanogen, SuperD, WG, etc. The hack is currently in Alpha stage but onnce the devs get all the kinks out or devices will fly. Knowing how the XDA works, it should not be long until this feat is mastered.