The official description-
http://www.cisco.com/en/US/technologies/tk648/tk361/tk438/technologies_white_paper09186a0080091cb9.html
The extendable keyword allows the user to configure several ambiguous static translations, where an ambiguous translations are translations with the same local or global address.
ip nat inside source static extendable
You might also want to define static mappings for a particular host using each provider's address space. The software does not allow two static translations with the same local address, though, because it is ambiguous from the inside. The router will accept these static translations and resolve the ambiguity by creating full translations (all addresses and ports) if the static translations are marked as "extendable". For a new outside-to-inside flow, the appropriate static entry will act as a template for a full translation. For a new inside-to-outside flow, the dynamic route-map rules will be used to create a full translation. Long explaination short; it allows you to have multiple NAT (port translations to be exact) statements to a single public address.
Hope that helps.
