About This Answer
A meditator describes shoulders and arms shaking and the head moving on its own during sitting, using the breath as an object and sometimes switching to the belly or whole body. Ajahn Dtun's advice, through the interpreter, is to keep the focus on the breath coming in and going out at the tip of the nose in formal practice.
Whatever happens in the body should be left to run its natural course without attention, even when one notices it. The movement can act as an obstruction: if the mind keeps going out to it, concentration goes only that far. With mindfulness and concentration that pass beyond the shaking, the mind becomes more and more peaceful and things sort themselves out.