These are rarely front-line serves as even intermediate players have little trouble attacking them if you use them too often. However, they are a great variation to spin serves, and if used a few times each game will often catch the opponent off guard. With experience, you'll get a feel for when and where to throw one of these fastballs at an opponent.
The keys to an effective fast serve are:
Let's look at how to execute the serve while doing all six.
Speed. This is the whole point. The problem is most players don't understand how to maximize this, or chicken out under pressure. To serve fast, you must:
Depth. You want to serve as deep on the table as possible in order to jam the opponent. Just as important is that if the fast serve isn't going deep on the table then you aren't maximizing the speed. A fast serve should, by its very nature, go very deep or you aren't serving it very fast. There's a simple way to increase the depth of the serve - serve faster!
Placement. All fast serves should go to one of three spots: wide forehand, wide backhand, or to the middle (the opponent's switchover point between forehand and backhand, usually the playing elbow). Fast serves that go to the middle forehand or middle backhand are just feeding the opponent. Make him move to the corners or make a decision and move to cover the middle. To practice this, put targets on the far side of the table and see if you can consistently hit them.
Spin. To maximize speed, put some topspin on the ball to pull it down. However, sometimes you want to vary this by serving with sidespin or a fast, dead (flat) serve, often at the middle. You can create topspin or sidespin by essentially slapping the ball with an upward (for topspin) or sideways (for sidespin) motion. You don't want to graze the ball too much with a fast serve as you won't get enough speed that way - that's how you create a spin serve. To serve a fast, dead serve, you need to contact the ball with a very slight downward motion, putting a very light backspin on the ball, sort of a downwards slap at contact, just as with topspin or sidespin. After two bounces on the table the ball will be dead.
Consistency. It's pointless having a fast serve if you often miss it. You need to get a feel for both the proper contact with the ball and where to contact it (both the height and how far behind your end-line), and practice it until it's second nature. Then make sure to practice the serve before tournaments and big matches so you can execute the serve under pressure. As noted above under Placement, one way of practicing the serve is to put targets near the opponent's end-line and see if you can consistently hit them. You might also put a target on your own end-line and see if you hit that when you serve, to make sure that your first bounce is near your own end-line. Also, while some develop the serve by gradually building up the speed that they can serve fast, you might consider the opposite - serve as fast as you can, and gradually slow it down to the point where you can get it to hit the table. Then work at controlling that pace.
Surprise. Your fast serve loses most of its effectiveness if the opponent sees it coming too soon. So set up as if you were serving your normal serves. Learn to do the fast serve from the same stance and starting with the same motion. The only difference might be that for a fast serve you might have to toss the ball back a little further so as to give yourself room to hit the ball so the first bounce is near your end-line, but if you do, that should be minimized. But your backswing should look the same whether you are serving fast or not. You don't want to telegraph to the opponent that the fast serve is coming.