刹那 is a transliteration of Sanskrit क्षण (kṣaṇa), the Buddhist unit of the smallest perceptible moment. Classical commentaries reckoned 75
刹那 to a
念, and several
念 to a single second - so
刹那 is poetically 'a flash too short to measure'. The
间 'between / interval' rounds it into the adverbial sense 'within that flash'.