It's based on the game theoretic idea that if your adversary has no way of knowing how many hidden partitions you have, then you have no way of proving to them that you've given them all your secrets.
As such, there is no benefit to you revealing any secrets under torture, because the torture would continue even after you've told them everything, therefore there is no point to them torturing you in the first place.
[0] https://en.wikipedia.org/wiki/Rubberhose_%28file_system%29