Fiddler allows you to implement a man in the middle attack against yourself to see what is inside the SSL tunnel.
Fiddler generates a unique root SSL certificate for your computer, and then creates a session certificate signed by the root. When you configure Fiddler to decrypt SSL for you it acts as a proxy between you and the destination, decrypting the traffic using the site's SSL certificate and then re-encrypting it using its own certificate.
Your browser/application will see an untrusted certificate error. You can get around this error by choosing to trust Fiddler's root certificate.
.Net Rocks did a great podcast episode at on using Fiddler to sniff your traffic