The word psychology means literally 'science of the soul' (the Greek psyche ψυχή means soul, not mind). Most cultures admitting the notion of soul (especially ancient/primitive ones) describe soul as a pool or lake in the lower belly. Indeed the word soul comes from the Old English sāwol which originally meant 'from the sea'. So the instruction: Mind in Dantien doesn't mean put your thinking mind in the belly but rather Connect with your Soul.