As with most swarm prevention strategies, the goal of this method is to create the appearance of more space in the hive. In most cases (well, in a lot of cases), the colony will have worked its way up into the upper brood chamber come March. In mid- to late-March when the temperatures have reached the upper-50's, open the hive and locate the brood nest.
If the majority of the broodnest is in the upper chamber, reverse the two brood chambers. If they are still spread pretty evenly between the two supers, give them another week or so. You really do not want to do this on the precipice of a big cold front when it's going to drop down into the 30's within a day or two of the split. The bees will have to cluster up and this will leave some of the brood exposed (and you'll end up with a lot of chill brood - not the end of the world, but you sure didn't help the bees any.)
Repeat the above process every 2 weeks or so, until just before the main nectar flow (mid-April or so in central Virginia.) When doing the reversal, it's a good time to move a couple of empty frames from the outside to the center of the upper brood chamber. Remember, the goal is to create space for the queen to lay.
As March comes to a close, it's frequently a good idea to drop an empty super on the strong hives, in anticipation of the early nectar flow.