打手亦是消手, the striking arm is also the neutralizing arm. LSDD means simultaneous, not block then punch. Put these together and it clearly means one arm, one beat, two functions.
You assume you can use 1 arm to deal with your opponent's 2 arms. In the following picture, you assume that your left arm can
- pass your opponent's left arm,
- pass his right arm, and
- hit on his face,
without using your other arm at all. Do you have any clip to show what you are talking about?
- When you punch,
- your opponent arm is in your striking path,
- you try to use your punching arm to neutralize your opponent's blocking arm.
2 situations can happen here.
1. force against force - Your opponent uses his left blocking arm to resist your left neutralized arm.
2. yield and borrow force - your opponent yields into your left neutralizing force, borrows your neutralizing force, add on his force, and use his back right arm to redirect your left punching arm away.
In both cases, to assume that you can only use your left arm to achieve neutralizing and striking, and also pass his left defense arm and right defense arm, it's not an easy task. The reason is simple. Your opponent can neutralize your neutralizing force too.