Membase has a method of adding new operations and extending protocol. This allows developers and contributors to experiment with new capabilities, then either add them to Membase as an extension or contribute it to the memcached core, as required.
The TAP protocol is one such extension which is somewhat special, and is proposed for inclusion in memcached 1.6.
Other protocol extensions include:
getl/unl: Get an item with a lock that has a timeout. It can then be unlocked with either a CAS operation or with an explicit unlock command.