Compressor manufacturers recommend turning the clutch prior to initial start-up of a new compressor. This spreads lubrication that may have settled over time and helps prevent potential compressor damage at start-up. This tool is designed to provide the user with an easy way to rotate the compressor shaft prior to start up if there is no Hex-end or nut on the clutch hub. Reduces potential compressor damage - less warranty work. OE recommended. Works on clutchless hubs.