It's nice to know that there's people that are putting the old posts to good use.

Anways the shifting of weight occurs in the motion as described by greynotsoold. You shift your weight following when you adjust your back foot, turning it in and making it flat.

There's a few more things that should be noted. Avoid telegraphing your hook by raising your elbow up to soon, instead it should be done at the last moment.

The other one is when you drop or pull the side of your body back. People do this to put more power into their punch but instead it slows them down and let's their opponent know what's coming. Your hand doesn't need to be cocked back at all, just make sure it's on the same level as your target when you're throwing it and whip it keeping your shoulder loose until you catch them. If I'm not mistaken, another point which Scrap brought up is to pull your right shoulder back when throwing the left hook.

If you want more power into it what you should do is set the hook up with your right hand which will get you set for a left hook. Be careful though when up against a fighter that is aware of your weight shifts. I think thst this motion works best right off of a slip.