This is a TSSC 02 Spring Compressor
Begin by backing off the preload collars as much as possible to relieve as much tension on the spring as possible.
The shock shown is not from a CRF250L but you get the idea. This tool sits on the spring and pulls up on the clevis. As you tighten the tool a gap will open up. You will need to push down on the bumper to expose the nut. When you can, insert an open end wrench onto the nut. The clevis must be held from rotating. Loosen the jam nut.
Remove the shock from the tool and continue unthreading the clevis.