BMA456 support total FIFO length 1024 bytes. If you set sensor with 600 bytes watermark level and FIFO headless mode. When ODR is 100 Hz, FIFO frame data(6 bytes per frame) will be filled to FIFO, FIFO watermark interrupt occured until FIFO length reached 600 bytes. This interrupt could triggle host MCU to read FIFO data. Whether host MCU reads or does not read data, FIFO data will be filled into FIFO every 10ms. If host MCU reads data when FIFO watermark interrupt occured, the FIFO length changes from 600 bytes to 0 bytes. If host MCU don't read data when FIFO watermark interrupt occured, the FIFO length will increase from 600 until 1024 bytes are full.
INT1 or INT2 could be remapped to FIFO watermark interrupt as a hardware interrupt.
Example code in github didn't demonstrate FIFO watermark interrupt to read FIFO data by hardware interrrupt. You could refer attachment example code by reading FIFO data with hardware interrupt.