Null is an end of cast sequence blocker that will stop the sequence from looping back to the start and being cast again. So when the sequence gets to Null it will no longer use any more abilities. So you let go of SHIFT at that point because the OPENER is over. This will allow the main macro to begin again.
@LEGION_MACROS great job! really enjoying it thanks! just small things what i observed that on Mythic ROTA Storm, earth, fire and Strike of Windlord not firing need it manually, could you give some love and recheck sequence or u left it intentionally for manual input? Thanks for feedback