He breaks people. The rule implies that, if he begins fishing upstream of you, he has every right to be there. If you get out of the water long enough to slow the progress of the upstream angler, let him fish through. The key, be reasonable. Be reasonable about your concept of the pool.
Little rivers tend to have little pools.
Sometimes when I am fishing even two anglers, I will space them, allowing each to fish his own pool, if the waters are strategically close. By contrast, a river the size of the Skagit may have pools a half-mile long, if you were to fish the whole thing. If you know you won't be fishing the lower waters for a least a half hour, and a guy walks up to ask if he can fish, let him. If you are looking forward to fishing that water yourself, and you know you are fishing iffy water, you should reel up and go down and fish the prime stuff yourself, but do not hoard the whole pool, simply because you happen to get there first. You just have the right to fish it first.