Short side and long side
The f-pawn splits the board in two. Towards the h-file there are only two files, g and h: the short side. Towards the a-file there are five: the long side.
The rule is to put the king on the short side and the rook on the long side. From there the rook can check the white king from the side, from far enough away that the king cannot chase it off without leaving its pawn.
In this position
Black's king on f8 has to choose. Kg8, to the short side, draws, and so does the immediate check Rb6+. Ke8, to the long side, loses.
After Kg8 Ke6, Rb6+ checks from the long side. If the king steps to e7, the rook goes back to b1, ready to check again, and after f6 the check Re1+ holds.
Why the long side loses
After Ke8, Ra8+ drives the king to d7, and White answers Kf7. Now the black king stands on the seventh rank between its own rook and the white king, so the rook cannot check along that rank, and the white king shelters in front of its pawn.
Remember
- King to the short side
- Rook to the long side
- Check from a distance
- Do not block your own rook