The same was a sculptor extracts a bust from a block of raw marble -- subtractive geometry.

Bitcoin's rules specify what isn't allowed and violating that breaks compatibility, but the system can become more restrictive without becoming incompatible.

Huge chunks of the space of possible inputs was set aside for futures use by allowing them in blocks without imposing any structure (while disallowing them for loose transactions so nodes won't inadvertently relay transactions which are invalid due to rules they don't understand).

In that sense all future features are already complete latent inside the system and to make them useful the community need only chip away at all the permitted inputs which aren't compatible with the desired new functionality. :)