It does nothing.
ATAR is a rank, determined by your aggregate (a score out of 500). This is determined through complex calculations. But what BOS gives UAC (UAC determines your ATAR in NSW) is your raw HSC exam mark and your moderated assessment mark (both before aligning).
Your raw HSC exam mark is simply the mark you get in the HSC exam before any aligning or option scaling. Your moderated assessment mark is calculated by your raw school mark and other factors such as the range and mean of HSC exam marks.
Essentially, at no stage does BOS or UAC look at your individual rank. Even though they need to know the highest and lowest ranked students so they can moderate your assessment mark, that can be done without actually ranking students, rather just listing marks from the highest to lowest. They don't say "oh, this student is ranked 8th, so we better give them this mark" - no, that doesn't happen.