u/Adept-Willow1332

Can anybody help explain what this little missing knob does homeless Abu Garcia easy cast baitcaster? See photo. This is for a friend of mine He's not really technologically advanced and doesn't really know how to ask the internet besides saying hey Siri and telling her to make a phone call and that's about it so when he doesn't know something and I don't know something he asked me to see what I can find since I know how to basically use Reddit lol. I tried to find out what it was through Google but I figured I would just ask instead of reading what they said and not understand it since I'm not a man of the fish nor some people say fishing pole people? Idk. Any help or explanation would be greatly appreciated and thanks in advance.