From the introductory blog post :
> All of the browser subsystems are present on your Kindle Fire as well as on the AWS cloud computing platform. Each time you load a web page, Silk makes a dynamic decision about which of these subsystems will run locally and which will execute remotely.
Then there's the Ars article  that explains some of what Silk's split architecture can do:
And the Terms & Conditions  make it clear that Silk can stand on its own two feet if desired:
> You can also choose to operate Amazon Silk in basic or “off-cloud” mode. Off-cloud mode allows web pages generally to go directly to your computer rather than pass through our servers. As such, it does not take advantage of Amazon’s cloud computing services to speed-up web content delivery.
When they say "split architecture", it sounds like they really mean it.