There are many points in kata where we execute double blocks or double strikes. The first movements of Pinan Shidan and Yondan are examples of double movements, as are the "split blocks" in the Naihanchi series.
We throw two punches to the side in the early sequence of Wankan and we execute two middle blocks near the end of Pinan 5. Naihanchi Shodan ends with two punches to the right. You can probably think of many examples in your own system.
The question is whether double movements should be executed simultaneously or slightly off timed. I have heard arguements in favor of and against both propositition.
I discussed this with my Sensei during my recent visit to Okinawa. We had also discussed this in the past. This post reflects what he advised on this subject and my application of his advice.
Of course, movements in any kata represent a range of movements, rather than just the specific movement performed in the kata. This also goes for double movements. Each of the movements (the right and and the left hand) represent a range of movements. As such, double movements can represent many movements (twice as many as a single movement).
But even if we limit ourselves to the set movements of a double movement, the issue of timing is still an interesting one. Take the first movement of Pinan Shodan. The left hand does a middle block (uchi uke) to the left and the right hand does a high block (jodan uke) to the front. Please not that I use the term uchi uke to refer to an "outward" block. Some schools reverse the "uchi" and "soto" designation. In any event, the first movement of the left hand in Pinan Shodan is a block to the left.
In my case, the answer depends on who I am teaching. For a beginner, it is easier to teach them simultaneous movements. The hands move together.
But as a student advances, and becomes faster, he begins to realize that there is a "dead" moment after double movements, a moment when it is very difficult to move his body or execute another movement. Double movements create "dead" spots -- gaps in timing. Such gaps can create an opening for an attacker.
For advanced students, I teach that double movements are best performed slightly off timed. To a casual observer, the movements will appear to be simultaneous. They will look like double blocks. But actually, they are a split second off timed. It does not matter much whether the right or the left is first, and in fact, this may depend on your direction and the flow of your movement.
For Pinan Shodan, my left hand is slightly ahead of my right. This is true for each segment of the three-double movements to the left: the two blocks; the cross; and the left punch. You might wonder whether the "cross", and the left punch are double movements. They are. Even when you punch, if you pull back your other hand at the same time, it is a form of a double movement.
Again, to a casual observer, perhaps even another Karate student or instructor, the double movements will appear simultaneous -- but they are not.
Your body can feel the subtle difference between a double movement (which creates gaps) and a set that is off timed. In the shudder or vibration of the off timed recoils, there is the potential for further dynamic movement.
Charles C. Goodin