Usually, disk encryption is done by creating a master key, which is then encrypted again for every user, using the user's password in OSX's case, that is allowed to access the disk.

This way, the user never can retrieve the master key and access rights can be modified all while keeping the master key and thus avoid re-encryption on the drive if a new user is added/an old one removed/one changes his/her password.

Downside: if the master key goes boom, so do the data, with no chance of any recovery.