It has a coupled rangefinder. This means you see a patch in the center of the finder which is imagewise identical to the respective part of the finder image, except that it is note really congruent, but slightly laterally moved. You thus see a dubble-image at center. At turning the focusing barrel this patch is moving laterally. The moment you no longer see a dubble-image, this part of the scene is in focus.
Or rather should be. Also there may be a vertical misalignment. Furthermore the patch may me be hard to see at all, as at least one mirror is involved and its reflective coating may have derteriorated over time. The rangefinder is the most prominent feature, but it is only a focusimng aid, you can focus without it with a bit of distance evaluating training.