Hi! Doesn’t the explanation in our documentation explain it sufficiently?
Hidden volumes allow to hide data in the encrypted volume. The data is protected with an additional password. Without the password the data existence’s can’t be proven. Hidden volumes are not setup by default so that their existence can be denied plausibly. The concept is similar to VeraCrypt’s/TrueCrypt’s hidden volume but with Nitrokey Storage the entire functionality of hidden volumes is implemented in hardware.