How does it work?
Run apps inside the yahoo mail  web ui,  these apps integrates with you inbox info, on its first run it will ask you if you want to connect your inbox files to the apps like facebook apps,
e.g. picnik, this app can open your inbox picture files mostly photos attached in your mail and you can edit or create directly inside yahoo mail.

Was it any cool?
It’s not, not yet, maybe a cool app will be developed that maximizes the inbox info on your yahoo mail, ahmm I’m thinking of twitter why not, “AGAIN….”, or some kinda timeline tool like plurk timeline, see you inbox message in timeline mode.

Just a caution?
Yahoo mail is a huge MEMHOG in firefox especially when web chat is open, the yahoo web chat uses flash w/c results to super duper memory usage, don’t use a yahoo mail web app with chat open, sign out of chat first.
I dont’t know why memory comsumption is that huge in firefox in IE its not that big though, Yahoo mail on firefox is notoriously slow and hangs much of the time for machines with small memory.
You can always try opening yahoo mail in google chrome.