I last used this feature probably more than a decade ago, but: you provide 2 passwords when decrypting. If the first password is the main volume, the second is attempted as a hidden volume. If both match, the main volume registers the hidden volume as free space but prevents writing to it. If the hidden volume doesn't match, the main volume will clobber the hidden volume.

So the main/hidden volumes really works like a duress: you might destroy your hidden volume while using the main one under duress, but that does not apply when using the main volume while able to additionally unlock the hidden volume.

If you are in a situation to need to worry about any of this, you're probably going to jail for one reason or another, anyways.