Hold down units are components used to hold down lengths of cable tray or ladder onto pieces of strut. Different trays and ladders tend to use different types of hold down, depending on their size and profile, but the overall principle on how they work remains the same:
In the case of ET3 tray, to install a pair of hold downs, insert 2 channel nuts (M10) into a length of strut, and line them up with each side of the tray. Then, hook each of the ET3HD brackets over the sides, and bolt them down into the channel nuts with a fastener. Use a washer between the fastener and hold down. Always install hold downs 2 at a time, on opposite sides of the tray or ladder. The length of the tray or ladder that you are installing will determine how many pairs of 'HD' units you will need to use.