When practicing zazen with the koan What is This? should one aks the question on the outbreath similar to susokkan or Muuuuuu? Or should one become one with the question deep from within the tanden without synchronizing it with the in and out breath?

You can place the question: what is this? On the exhalation. Like following the Mu koan. But it is not a fixed way of doing it. Here it is all about Kufu – creatively finding out what works best for yourself, how you can stay in deep concentration and application.