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.
This is interesting. In reference to this, I remember once being told that "being between Heaven and Earth" was a reference to something in traditional Japanese culture that as human beings we dwell in the sky/space literally (with the emphasis that we stand upon the Earth while being in the air). Yet, counterwise, I recall a reference to this that traditionally this notion considered a bridge suspended between Heaven and Earth on the horizontal (with Heaven and Earth beside us; not above or below).
Hence, I have a further question. What is standing between Heaven and Earth in a practical manner? Furthermore, what characterises Aikido lacking this? Do we do stand between Heaven and Earth consciously or are we standing between Heaven and Earth as a consequence of our liveliness and we should be aware of this?