Think flexible-sized partitions. Then forget it again because it is too wrong.

The one really nice scenario this opens up that is discussed in the pull request linked in another comment:

  1. Server A has an encrypted dataset in pool foo, currently not decrypted
  2. A can send full or incremental streams from that dataset to server B
     without decrypting
  3. B can receive those streams and import them as encrypted
     datasets into the pool without decrypting or really ever having
     even seen the keys
  4. Server A can restore from B as required
  5. The owner of the key material can log into B and unlock
     the dataset as if it were on A

This is a really nice, accessible way for encrypted remote backups.