A strut spring compressor is a tool used to squash coil springs on McPherson strut assemblies. Coil springs hold the weight of the vehicle and store massive energy. When the strut is off the car, the spring remains compressed under hundreds of pounds of pressure by the top strut mount nut. If you remove that nut without compressing the spring, the mount will fly off with lethal force. The spring compressor uses two threaded rods and hooks to compress the spring safely before disassembly.
Safety is the most critical part of using this tool. You must place the clamps on opposite sides of the spring (180 degrees apart) to keep the load balanced. Grease the threads of the compressor rods to prevent them from seizing under tension, and tighten the rods evenly, switching back and forth between sides. Never use an impact wrench on cheap spring compressors, as the high vibration can crack the cast iron hooks.