This precept, as I see it, could have a few different meanings that apply not only to Karate but to life.
When you are learning something new, even if you have experience in something similar, you need to clear your mind or empty your cup in order to put more in. If you go into Karate training thinking that you already know something, you will take the chance of not learning the concept or technique correctly. Any preconceived thoughts or notions will limit your ability to open your mind and learn.
Another meaning could be where you don’t think, you just do. The could apply to the automatic execution of a technique without thinking about it because you have trained it so many times. Have you ever been walking along, not paying much attention to anything around and suddenly someone throw a door open in front of you and who shift to the side and downward block the door without even thinking?
It could also have a more Zen type meaning where each technique is the only focus for that moment. For example, when performing your kata, the technique that you are executing is the only thing on your mind in that moment. You aren’t thinking about the next move, you are only thinking about the current move.