2.3 litre exhaust flapper

[88 Capri]

I was doing some reading and noticed that I should have one of these. I brought the exhaust pipe to the bench and noticed all that is there is the shaft.
My questions are what purpose does it serve, do I need it and how would the engine run without one.
I'm curious on my last question because I had trouble getting the engine to idle below 900 rpm and was thinking it might be something with backpressure. This is my first inboard boat so please excuse my noobness 😊
Thanks,
Jim

[Scott Danforth]

it is there to prevent hydro-locking should your motor ever run-on (common problem) or you back up quickly

[88 Capri]

Oh ok makes sense thank you.