"The day the heaven sways with a swaying." There is a transposition here: "Indeed, the punishment of your Lord will certainly come to pass" upon them "the day the heaven sways with a swaying," meaning: it moves with a movement.
﴿يَوْم تمور السَّمَاء مورا﴾ فِيهَا تَقْدِيم: إِن عَذَاب رَبك لوَاقِع بهم ﴿يَوْمَ تَمُورُ السَّمَاءُ مورا﴾ أَي: تحرّك تحركاً
