You start with an actual store of value that gets degraded over time through the people holding power over it (think mints). The coin starts with a high silver or gold content and with ever economic woe the ruler finds themselves in a new coin gets minted with some copper and tin added. Or just prints more.

Power comes into play as to dictating what currency people are forced to use. That the old coins had to be handed back in or dictating the adaptation. They implode when there is nothing backing it what so ever. At its core its Greshams law at work, with currencies or with other store of values.

If i am not mistaken Graeber has quite a few examples of this mechanism.