Being "partition tolerant" just means the other aspect holds: CP means "consistent in case of a partition", while AP means "available in case of a partition".
A CP system cannot be available in both partitions, since by definition the two partitions cannot communicate (otherwise they wouldn't be partitions) and thus it's logically impossible for two clients in two different partitions to affect a shared state consistently. Thus at least in one partition the service will be unavailable.
Forcing all state changes go through a single "master^Wmain" node (for some key/shared) is a simple way to achieve CP.
This main node can change over time, but in case if a partition it can never move to side of the partition which has no quorum for a master election.
