All I can tell you is what I tend to do and comment accordingly, there is likely to be other techniques, I don't pretend to be the final authority and I'm not an instructor.
Also, dispite having skiied for more years than I'm willing to admit my experience is almost entirely on piste... and hard piste at that. I couldn't tell you how much of this, if any, is applicable in powder. Did quite a bit of racing when I was younger, too.
Basically I tend to initiate slightly forward and go to the center when I lay into the turn looking to bend the ski in a uniform manner. While edges are fully engaged and at the point of maximum G's in the turn, weight can come back a little to load the tail of the ski for some added pop to accelerate you out of the turn. BUT make sure you hold on and get back over them immediately or you'll get ever further back until you're on your butt. If you try this going too slow the ski won't bend enough to give you the rebound to shoot you out of the turn. You're trying to produce an effect similar to a watermelon seed being spit out from between your fingers when squeezed.
Allot will be influenced by the ski you're on. If it's a mushy begginer's ski there may not be enough guts in the tails to get any pop at all, you might just induce a skid. Another matter if it's a hot carver with a race pedigree. These skis tend to have a stiff tail and the bindings mounted further forward giving you more tail relative to the size of the ski. Beware, however, that these skis WILL give you that acceleration... in fact if you over do it they may just jet out from under you and it's "GAME OVER".
In the right dosage it's that tendency for the skis to jet that you want to learn to control and take advantage of. You'll have to acquire a feel for the sweet spot of your ski and the not so sweet rearward point where things start to get aggresive, then experiment.
The best resource to see this technique being exploited fully is to observe how a racer shifts his (or her) weight while moving through the gates. On this site you can find some sequential shots of racers and it's pretty easy to see when they're employing the tails to get that kick you're after.