Bunkai, the application of kata, is an important aspect of our karate training enabling us to employ the techniques of the kata against an actual attacker.
"In Bunkai practice our real opponent becomes actual and is more easily observed and appreciated" (Hanshi Scaglione, Building Warrior Spirit ("Green Book"), page 22).
This allows us to develop our sense of ma-ai, or distance -- to "intuitively establish the correct distance between oneself and one's opponent" (Okinawan Karate Question and Answer ("Red") Book page 62). Bunkai also trains both the person performing the kata and the attacker to use proper karate techniques in a fighting situation and to focus on hitting the target, in this case specific vulnerable striking points of your opponent (see diagrams on pages 39/40 of the "Red Book").
Bunkai is traditionally practiced with one or more training partner/s. It is recommended that we all have at least one training partner with whom we practice bunkai on a regular basis. One partner performs the kata in response to attacks by the other partner. It is important to note that the attacker is always attacking -- never employing blocks. So an attacker would throw say a chudan-zuki (chest punch) as the person doing the kata throws the same in response -- the attacker would never block the chudan-zuki thrown by the person doing the kata. In bunkai practice the attacker is also challenged to use proper karate techniques and stances to deliver the attacks rather than wild "roundhouse street punches" etc.
It is suggested that in the beginning you and your training partner practice bunkai at half speed and power, as you figure out the ma-ai, and the applications to the specific moves in the kata. After many repetitions at half speed and power, you can start to increase the speed and power until eventually, after years of practice together, you can demonstrate the bunkai to the kata at full speed and power and full commitment.
"Full commitment is developed. This type of commitment should neutralize an adversary with one blow." ("Green Book, page 22).
Bunkai can also be practiced on one's own, using the heavy bag as your opponent. Here you position yourself so as to be in the proper ma-ai to execute the first move of the kata. After you execute the first move, you then reset your position and stance for the second move, and so on. This method also develops one's sense of understanding of the kata as you are forced to figure out where to set up and how to execute each move. The other advantage of the heavy bag is that you can deliver at full speed and power, without risk of injury to your opponent, something we never want to do in the dojo.
"Proper attitude is the absolute necessity...so as to preclude serious injury and also for the development of mind and body" ("Green Book, page 22).
Sensei Des Chaskelson