This has always been the way it works. By looking at WHERE the already counted vote is coming from (precincts put in all their counted votes at once, so they are done counting when they submit), people can determine what's left to count. You can then match that up with voting registration records to know if areas left to count are predominantly GOP or DEM, and then look at the turn out in those places. If it's heavier than expected, well... there you go.

There have been almost no races ever called that switched back. The only prominent one in my lifetime was Bush versus Gore, when it was called for Bush, but the count in Florida kept edging closer and closer, and it switched back and forth after that. Gore ended up ahead, and probably would have been the winner if not for Supreme Court intervention when they were doing the final hand vote counting. The previous count of limited districts gave Bush the state by some tiny margin.