Essentially this move is a sidestep. Assuming an orthodox position you take a step back 6-10" (This is so that you don't over reach), then in one fluid movement you quickly slide your right foot forward to the inside or outside position, slipping their lead and shifting your weight to your right leg. Once the weight has been shifted to your right leg you can bring a hard left hand to their chin or midsection. To get back into position just pivot and turn your body and follow up with a straight right. This move can be easily practiced on any of the hitting bags, especially the heavy bag.

On my heavy bag I have all the targets marked, and to practice the drop-shift I'll just give the bag a push and hit it while it's coming toward me. I can practice side-stepping to the inside, or outside, and aim for the chin, solar-plexus or heart.

I think against a real opponent it's important to make them commit to a straight lead. My best bet would be using clever footwork (quickly move forward then backwards) combined with a feint to create a false lead to draw them in. Once they have committed to a straight punch, you can take a step back as much as much as is required and drop-shift to avoid their lead and then land your counter. I can imagine that this can be combined with many set ups even effective with the jab.

If they don't lead or commit to their punch they can move away in time. The only time I was able to see the drop shift attempted was in the Miguel Cotto/Zab Judah fight. I think it was in round 3. Judah switched southpaw and tried landind a left to Judah's body. Cotto's timing and judgement was off and Judah moved away from him and his stance would have blocked regardless.

The drop-shift is nifty counter when done right but probably shouldn't tried twice in a fight.

I was curious to when this move came about and I stumbled across the Fitzsimmon's-shift in my readings. Although the move was around prior to him using it, it was his knockout over Corbett that brought the move such notoriety. From the descriptions that I have read I believe that the Fitzsimmon's Shift and the Drop Shift are one of the same.

I have found a video clip of this fight if anyone would like to check it out.

The Footage: http://www.youtube.com/watch?v=Zbe7nnnhNfc

I've heard of other champions such as Benny Leonard and Flash Elorde pulling it off, but I haven't seen those clips for myself.