This is because Eyn Sof, blessed be He, encompasses the Sefirot as seen in the prophetic vision both above and below. The light flashes from above and descends, spreading downwards. As it spreads, so it becomes weaker, and when it reaches the bottom it returns to the Source below – Eyn Sof, Who encompasses below. Then the light goes back and flashes from below upwards, following exactly the same path. Thus “the Chayot were running and returning like the appearance of lightning” (ibid.).
This shows the absolute control and perfection of Eyn Sof. For everything emanates from Him, and He is the end goal of everything... The whole point of this image in the prophetic vision is to show that the Sefirot are not something outside of Him in any way, for nothing exists against His will, but that His light shines in this way (within the Tzimtzum, like lightning) until it disappears. In other words, what is concealed (in the Tzimtzum) is the perfection that was in Him from the beginning (before the Tzimtzum), just as it is now (in truth, even in the phase of concealment, within the Tzimtzum, for even within the present concealment, His perfection is really in control), just as it will be at the end.