The decompressor is a mechanical device inside the engine that holds one of the valves partly open during part of the compression stroke, when the engine is turning at cranking speed. It is just about universal practice to have an automatic decompressor on high compression mower engines (this mainly means overhead valve engines) and is commonplace on the larger mower engines such as yours. I do not know whether your engine has a decompressor or not. The purpose of the decompressor is to make it easier to turn the engine over for starting. There are several different methods used. Most small Briggs engines just have the intake valve stay open 0.010" during much of the compression stroke, whether the engine is starting or running. (Not much gas leaks out through that small opening in the milliseconds of the compression stroke when the engine is running fast, so Briggs found it to be a practical way to make their engines easier to start. The feature was called "Ezyspin".)
