Not knowing the engine you are working on, I say you might have ACR on the camshaft that has failed. I have worked many Honda and Briggs engines where they kickback when these fail. Nearly broken my wrist on a Honda GX390 that kicked back while using the recoil starter. I did dislocate a couple fingers. I have since learn to go slow when first trying to start a non starter.

EDIT: ACR = Automatic Compression Release

Last edited by AVB; 13/11/19 10:27 PM.