The wording of this setting is kind of weird and doesn’t really make it very clear what it’s for. As far as I can tell, it really means “automatically track topics I read for at least _______”. It’s also a bit strange, behavior-wise. The length of time I spend reading a thread depends on the length of the thread, which has absolutely nothing to do with whether or not I’m interested in tracking it. It took me a few instances of getting inexplicably signed up to follow specific threads before I figured it out.
I’ve turned it off and my suggestion is that it should be turned off by default or junked entirely. It’s a very odd criterion to use for subscribing to a thread.