So for example, say you use the hidden partition feature. You have two encrypted partitions, one with your really secret data, and one other with some other data that you don't mind if your adversary gets hold of. Under duress you give your adversary the password to the latter.
In the UK, the prosecution can argue that the data you gave up is not all there is, and you are holding some back. As evidence, they can point to things like lists of recently accessed files that seem to point to data on that hidden partition, or the fact that the data you did give up has not been touched in a very long time and there is evidence that you were using the computer recently.
It's not a given that they can prove there is more data beyond a reasonable doubt, but you do have to be careful to avoid mistakes that can give them what they need.