According to the original IOTA post on bitcointalk, partition tolerance is not only defined as the ability for the IOTA transaction graph to resist the reversal of large sets of transactions, but also to allow it when needed by the network:
Blockchain-based currencies are unable to survive long-sustained partitioning of the network because this may lead to reversal of a large number of transactions. It is also impossible to initiate an intentional partitioning in cases when it is required.
(emphasis is mine).
These are presented as pros of the IOTA system. I.E. With IOTA it can survive a long-sustained partitioning and yet it is also possible to partition the network when required.
When it is required to initiate an intentional partitioning of the Tangle?
How can the Tangle be both resistant to partitioning* and also very much able to be partitioned if deemed necessary?
*By 'resistant to partitioning', I mean 'able to survive long-sustained partitioning of the network without the reversal of a large number of transactions'.