The Nakago is the tang of the Katana, the extension of the blade into the handle. But this Nakago hides many secrets, like many other parts of the Katana.
Poor quality sabers have no real tang, but a simple metal rod attached by dots, which makes the whole blade fragile. This is another reason not to select an inexpensive Katana made of poor materials.
On antiques, you will often find a signature of the blacksmith who made the blade. Of course, for this kind of sword, it is usually a few thousand euros. Besides, this part of the blade must remain intact at all costs.
The signature is not the only thing that is hidden on the tang of the Katana, indeed there are also on the oldest and most traditional swords holes that were made for the Mekugi
Problems of Nakago
The problems that can be often found are a very thin artificial silk that will not resist to shocks or a badly shaped Nakago. This causes the shocks to be transferred directly into the katana's tang, which may eventually break.