So the enunciation of a-u-m supposedly replays this eternal, repeating cosmic process: starting, continuing, and then coming to an end. This process starts anew in each yuga (yuga = any Hindu-Buddhist era, each of which lasts many thousands of years).
According to the Yajur Veda, the svastika is also a representation of om. So when you write a svastika you are essentially invoking the underlying sacred principle in this universe.
Hence, the svastika is an auspicious symbol, just as “om” or “aum” is an auspicious sound.