The I/F trigger stands for 'Interface Trigger'. It is a device used for immersion testing and allows the user to define a gate that will detect any signal that breaks the gate threshold and reference this signal as zero. All signals that then appear later in time will be measured correctly against this signal. Using this setup means that the calibration procedure is not necessary but be sure that the velocity is entered correctly. Using the interface trigger also allows the user to measure the water path which is useful for recording near surface as well as far surface topography. I think a step by step procedure would be better served outside the forum, so I will email you directly.