Okay Paul, let me try an explain my understanding of this.
This checkbox is a flag. Once you've checked it, you have switched on that get replies flag for that thread. If you have ten posts in that thread and checked every one, or just one, you have switched on that get replies flag.
To switch that flag off, you have to uncheck one checked checkbox. You can either go through the whole thread looking for a post that has that checkbox checked, and uncheck it, which is time consuming and crazy let's be honest, or you can go to any one of your posts, and if the checkbox is unchecked, check it, post it, wait a minute or so, go back in, uncheck it and post it. It is this action of unchecking a checked checkbox that switches off that get replies flag.
Think I've just worn out the c, h, e, and k legends on my keyboard, lol.