Here's a more detailed explanation quoted from Art on the Yahoo group
"MPG Shuttle mode. (Yes, its finally here!!!)
I really like this one. Run a program, during the run press the Shuttle
Button. The program will ramp to a stop (Deceleration time may be set int he
config/state screen). Now turn your MPG, the tool will move with your
turning of the wheel and execute the program slowly. Once your past the
feature you were nervous about, hit the shuttle button again and the program
will ramp back up to speed and continue. You may only go forward, but I did
allow you to turn the wheel in either direction just in case your right hand
gets tired playing with this feature. (It is VERY cool, I can see why this
is popular, and have no doubt it will be popular in Mach3...).
Whatever JOG mode you are in will switch to MPG mode when you press
shuttle, but it will revert back to what you had selected when you exit
shuttle mode. Please let me know what you think of not only the way it
works, but how well it works for you. Small tweaking may be necessary on my
part. (Though my mill and I love it.. )"
One thing he didn't mention, is that Mach3 has a reverse mode, and if you are in reverse mode, the shuttle will work in the opposite direction as well. It just can't go forward and backward at the same time.