When you hit an SSL site, the remote server (the one you're browsing) presents a number of certificates. One for the actual secure domain, and one or more for the CA that signed that certificate (sometimes more than one because the CA's have intermediate keys. A key which signed a key which signed... etc)

The CA itself is not a party to this exchange, they only provide the end product - a signed certificate.

If you trust the CA (in your browser config), you also trust by extension every certificate that CA has ever signed (barring revocation lists).

Thanks for the explanation, I was under the mistaken impression that communication took place between the CA every time a server was visited.