The simplest of the Six Simple Machines these are very versatile and essential if the seized shaft is in a threaded bore in a free moving assembly... as soon as the shaft starts to move tension between the attached surfaces will be released and it will start to turn. You can use a Crow Bar as a Wedge or a Lever against a nearby fulcrum to prevent this movement. You can also use a Crow Bar when a shaft is rusted into a tube and you need to crack the corrosion and lever it out.