Fast-twitch muscles

Is it possible to train fast-twitch muscles? I think I've heard some people say no. I saw this and it got me thinking:
http://www.gsprushfit.com/dvd/explosive_power_training/

The Jiulong Bagua forum has been talking about deadlifts and squats recently and my stomach has been changing size in the wrong direction. I've been thinking of doing more weight training, not that I have the time nor the energy...

It is possible to train fast-twitch muscle fibers, imo. Traditional weight training exercises won't really train fast-twitch fibers, but will make you stronger in general so the fast-twitch motions are stronger. Plyometrics (as stated in GSP Rushfit) are the best for fast-twitch training, I believe. Shadowboxing with weights would be a good way to start. Gradually get up to the point where you are shadowboxing with 15 or 20 lb dumbbells in each arm; you'll punch fast after that.

Recently I've been doing hard movements with ankleweights attached to my wrists, kind of simulating Shaolin iron ring training. I find that works tongbei fajin very well. You can also do sets of jumping or clapping push-ups, then slowly add weight to your back via weightvest or some device. The key to training fast-twitch is adding weight on a bi-weekly or monthly basis to plyometric exercises. They're not the most fun thing in the world, but they'll give you lots of power and speed in your movement. Just a few cents from me.

Is this what you're talking about when they lift the weights real fast in this video?

Exactly. Those are the kinds of drills that will build fast twitch.

What does is fast-twitch training good for? What does it help with?

Fast-twitch training trains the muscles to strike fast and hard, both speed and power simultaneously. The opposite training would be slow-twitch fibers, for things like endurance and strength. Isometric training is slow-twitch oriented. An example of slow-twitch training would be pushing against a wall for an extended period of time, or super-slow sets of traditional weight lifting drills like curls, bench press, lat raises, etc. Also, running, swimming or any endurance training would be slow-twitch oriented.

Fast-twitch will give you that ability to explode fast and hard. Slow-twitch will give you the ability to maintain the same level of strength over a given long period of time. It's ideal to train both. Here's an example for training against the heavy bag: if you train to have good fast-twitch fibers, you'll hit the bag fast and hard. However, if your slow-twitch is lacking you won't be able to last long at that intensity, and your power and speed will decrease rapidly. IMO, I think it's best to train fast-twitch muscles for long periods of time vs. short bursts, therefore your power and speed maintain the same level throughout. Hard movements in qigong would be an example of intensity/fast-twitch training, especially if you add weight to your wrists while doing it. The exercises in that video are also excellent for producing great fast-twitch fibers.