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Then should sempai say nothing when practicing, only demonstrate?

Yes, a verbal explanation is not the best. There are times when it is necessary, but it should not be the first impulse. If you talk too much, you are not a teacher, not a sempai. Students mistakenly try to understand something with their heads rather than allowing their bodies to experience and learn it.

One aspect of practice is to help purify your heart. This is done by action, not by words. Helping others and cleaning the dojo is an integral part of this process.