Stolen directly from Wikipedia. Not entirely sure that it helps, though.
In the First/Second and Second/Third Division, while the champions are automatically promoted and the bottom team relegated, there are playoffs of the second-bottom teams against the second, third and fourth placed teams from the league below. Home and away ties decide semi-finals and a final, and the overall winner plays in the higher league the following season, with the loser in the lower league.
Got it. There's only one automatic promotion place, and the bottom team gets relegated. So the second promotion place is decided between the three runners-up in the lower league and the 2nd from bottom in the higher one. Meaning that, if the latter wins the overall competition, there is no second promotion and relegation that season, they just keep their slot.
I hope this helps. Talk about making things complicated.