Update:
2007-08-20:11.11p : Changed the title to "Reverse Beat: Whiff Canceling" and changed the words in this post accordingly
2007-08-30: Changed the thread from "Jab Canceling" to "Reverse Beat: Jab Canceling"
Introduction
The Combo system of Melty Blood works in such a way that, provided that you are hitting the opponent, any normal hit can cancel into any normal hit. (ex: A cancels to B, C cancels to A) On the same token, once you do a move, you can't repeat that move in a single string once you used it. This applies to hitting the opponent whether he is blocking or not.
There is a slight different to hitting an opponent that is not blocking and one that is blocking. This will lead to 2 different kinds of options depending on the situation
If you manage to hit an opponent that is not blocking, you have the additional ability to "Jump Cancel", which means that you are able to cancel a normal move with a jump. It is well known when doing air combos from any sweep to another normal hit, you can immediately jump up in the air for pursuit. This is a basic concept of "Jump Canceling." It also means that you can hit a standing opponent with a B and jump right after word. Looking at this further, this brings along other options of "Counter Punishing', catching people in the air, or as other means of mix ups.
Most of the time though, before you will be able to land that hit, the opponent will be blocking, and that will limit your options with the strings that you will be able to use against your opponent. That means you will be strictly limited to canceling moves with a special move or waiting for something like a whiffed sweep to recover, which leave you screaming to the opponent "Hit me". Looking back to the concept that any normal move can cancel to any normal move, if you are met with this situation of hitting a blocking opponent, you will want to whiff something that gives the least amount of recovery. This leaves one possible option, the Jab A.
Whiff Canceling
Whiff Canceling refers to the use of whiffing a standing A in mid pressure against a blocking opponent. The reason why standing A is a good option is because a majority of moves from ground encounters are done using crouching moves. In most cases, standing moves will not be used unless in the middle of a combo. This leaves the standing A an available option to recover quickly since it isn't used much otherwise. In a combo string against a blocking opponent, a standing A that is whiffed will bring you back to a neutral state quickly. That means you can rush in and do every single one of your normal moves, then whiff an A, the do all your moves again.
This also give the player different options that are not available otherwise. Any move that otherwise has an extreme recovery can have an A whiffed right after for a quick recovery. Sweeps can be spammed against a grounded opponent. C's can be Whiff Canceled, only to follow up with another C. One can even throw an opponent after a Whiff Cancel, provided they are close enough to do so. Also, Depending on the situation, other moves can be canceled into provide that they haven't been used yet in a single string
Drawbacks
While this concept may sound like a godsend at first, there are some drawbacks with Whiff Canceling. First of all, a move can only be canceled if it actually connects against an opponent. This means that a normal move like a laggy C attack that doesn't connect will be just that... a laggy C attack. Whiff Canceling only assures that you can continue any pressure string used against an opponent. It will not prevent an opponent from blocking, countering, or shielding you. This concept is only considered as an additional means to use against an opponent. Also, depending on the character, some normal moves cannot be Whiff Canceled, or have other specific follow ups, thus not allowing the option to Whiff Cancel. Each character has its own use of this method, therefore using this tactic may take some experimenting or getting used to.
Conclusion
Aside from the drawbacks though, Whiff Canceling offers a solid option for recovering quickly in mid combo through the use of normal moves. It lessens the chance of moves that scream "Hit me" from staying out longer then it should. It is also universal from character to character, so each character has a means of using it in their own manner. While the use of this tactic may not win all your matches, it does present as a tool against players of a higher caliber. After all, the best way to attack or defend is to make sure that you are ready for it in the first place.
In closing, I'd like to say good luck on your matches. I hope this helps your gameplay.