I have seen several very good explanations, but I did not fully understand or feel it personally until I saw and tried this several months ago. I planned to post this when I got home from work today, but knightsb beat me to it 
Noon nails it here:
See if this does anything for you... who doesn't like a little experiment!
Stand on your rear leg facing a wall. ( you can pretend to hold a bat ). Your front leg knee lifted up a bit, front foot off the ground. So you are balanced on your rear leg. From here you can do several different things to coil.
A) Bring your front hip back while pushing your rear butt cheek directly behind you. Your front knee will angle rearward and you stay balanced over your rear leg.
or
B) Don't move your front hip or knee back, and just sit into your rear hip and push your rear butt cheek directly behind you . Again you stay balanced over rear leg.
or
C) Don't move your front hip or knee back, and try to move the rear hip forward and around the back so that it is directly behind the front hip. This should get you coiled in the rear hip while also shifting your center of gravity such that you are no longer balanced over your rear leg and begin to move in the direction towards the pitcher.
I'd say A) is typical of those who use their front leg lift to push back and coil the rear hip... that's coil back...not good. B) is better than coiling back but usually results in a coil then hold and move forward action. Not ideal in that any time you have to hold or pause and action that is breaking the flow and not in sequence. C) is coiling forward or forward by coil.
For extra fun after you have done C) a few times try to have the hip action described while keeping your belly button pointed at the wall in front of you. (i.e. as your hips "close" while moving forward, don't let your belly button turn back) . I say this assuming that as your were trying it before, as your hips "closed" your torso above also closed and belly button and shoulders pointed rearward as well. So try it again with the intent to keep your belly button from turning back. Note what that does to your shoulder line as you move forward. You should get some lowering of the front shoulder while keeping the shoulders in line with the pitcher.
For extra extra fun, after you get some shoulder tilt happening from the belly button action happening on top of the hips coiling, put a bat in your hands and let them be reactive to the shoulder tilt..... when all said and done you may have barrel "tip" happening, with downward sloping shoulders, as you move forward without actually trying to "do" any of that... you were just coiling the rear hip behind the front hip while keeping your belly button from turning back.
Then you can feel the reptilian love of the Big Gila for yourself!
Original thread here at BBD (have to be registered to view).
