Two of the most important aspects of any physical engagement are timing and distance. Timing is something that can be broken down and trained. It's a skill as well as an innate ability. Maintaining the proper distance helps timing, in that it gives space (time) in order to react.
Timing can be broken down into three general types:
This is a common moment in time where you accidentally find yourself in the right position at the right moment. Your distance is correct, the opponent puts himself there for you. This type of timing cannot really be trained. It is something that is incidental and difficult to reproduce. When it happens, you often times don't know how or why. Often times, the accidental is not pretty, it's not in form, its just there. In the beginning of your training, you will find this type of timing taking place more often than the other two.
Intentional timing is planned. This is something that occurs with preparation or just simple awareness. Its a conscious decision or planned action. Great examples of Intentional timing are a progressive indirect attack, one where the opponent gets set up. This involves fakes and feints to get him where you want him.
Example: After striking low, you notice he drops his hands to block. Using this knowledge, you throw a low fake to reproduce that reaction and land a strike to the head.
Another instance of intentional timing is when you see the moment and are able to seize it. This is plain awareness of the distance, what it takes to strike, and seeing the openings.
Example: He steps in to hit, you see he dropped his rear hand, then you strike where he's not covered.
Intentional timing requires being in a relaxed state, where you can think clearly and see as things progress. The best way to work on this type of timing is sparring. You need to do it enough where you're comfortable and actually able to work. A common mistake in sparring is to make it about battle and winning. This type of sparring does nothing to advance your skill. Remove the ego and concentrate on applying.
Intrinsic timing is the ultimate goal. This is where Bruce Lee talked about "It hits all by itself". Its perfect timing that just happens on its own. The difference between this and accidental timing is that it's clean. You know why it happened, you know how it happened. You just didn't have to make it happen.
Intrinsic timing is the result of lots of training and practice. The more you train, the more often it comes out. Anyone who has played an instrument knows the feeling when it's time to free-form solo, things just come out of you from somewhere unknown. In Star Wars terminology, it's THE FORCE. It's the ultimate skill.