FSYNC is just an input signal that is used to generate an interrupt from the sensor. The sensor stores acceleration data in the FIFO for the duration of the frame (between two FSYNC edges) and MCU can go and read the whole buffer in one transaction.
That's it, there is nothing special about it. You could really just use VSYNC interrupt from the sensor itself for that, but doing it though the sensor allows you to play with polarity and generally makes writing drivers easier.