As far as I remember sensei teaching, he could refer to experience 'being between heaven and earth' every time you do a technique. I'd say it is quite difficult.
interesting that. in vietnamese there is a phrase that has literal translation of "standing with head holding the heaven and feet stamping on the earth". it was an adaptation of the same saying in chinese. what it meant was "be proud and fearless". my guess is what he meant to stand with good posture,i.e. head up, spine align, chest out, breath deeply (like a proud person and not stooping) and move without fear as though you owned the mat. just a guess, mind you. i could be way off base on that.