The highest expression of Aikido is to be able to do all waza effectively regardless of the situation. In heat, in cold, before 100 silent eyes, in the midst of negotiations –with shoes on. Anyone who does not practice talking while doing waza will be at a disadvantage in a real life situation. Sure you may be able to injure someone successfully, but how well can you achieve harmony? Our mouths are one of the greatest tools we have for harmony. They are also extremely useful in leading a personfs Ki. If you get into the habit closing your mouth in a conflict or physical situation, there is nothing to do but go to fisticuffs. If you can continue talking the whole time before, during, after a waza and through the Osae, you will find that you may not even need the waza in the real life situation. According to modern criminology and Police Training, the longer a suicide attemptee is kept talking, the lower the chance that he will commit a violent act to himself. The same goes for terrorist situations. The longer you keep them talking, the better the chance of survival for the hostages. So talk before your waza, during your waza, after your waza, about the weather, about the waza, about anything, even be silent. Just remember to develop Zanshin, or continual awareness during your waza or you will get whacked upside the head when you start to talk. Do not let your Ki to be led away during the conversation.
Talking also helps you to remember to breath regularly during your waza. We have a natural tendency to tense up and hold our breath when we try to work hard or receive a hit but talking will help alleviate this.