The first command is feed: that allows you to tell MSN Search that you want to search for material just within feeds it has indexed, rather than across web pages and other documents. It's similar to existing filetype commands that let you limit searching to things such as Word documents or PDF files.
How about an example? Try feed:"hurricane katrina", and you'll get a list of feeds that have that exact phrase within them. Look at the first page listed, a feed from the National Hurricane Center. In the cached copy, you'll see that phrase appearing in the feed.
Keep in mind that only text in a feed is indexed, and that may be different from the text in a post referenced in a feed. In other words, some blogs and other sources don't send out the full text of their posts in feeds. In other cases, some people will write custom descriptions so that what's in the feed will be different than what's on the page.
Overall, the feed: command isn't that wonderful, though it's still nice to have. Use it if you're trying to narrow down a mention that may have happened in the blogosphere, since many blogs have feeds, so this is a way to drill into that subset of content. But plenty of non-blog pages also have feeds, so it's not perfect. Plus, since you're only looking in feeds rather than the full text of post, you might miss items you are interested in.
The second command is hasfeed: that's supposed to bring back any page that links to a feed that has those words in the feed content, from what I understand. That's not what I found, however. Running hasfeed:google brought back the Google home page first, and that page has no feed on it nor any links to feeds that I can see.
Robin's article has a few examples he was sent by MSN, and it seems like using hasfeed: with site: works better, as a way to see if a particular site offers any feeds. For example, hasfeed: site:searchenginewatch.com does bring back pages within the searchenginewatch.com domain and subdomains that link to feeds -- since we have a link to every feed on every page.
A better combination shown in the examples Robin was sent is something like feed: site:searchenginewatch.com. That brings back only feed content from within the searchenginewatch.com domain and subdomains -- and it did catch all the major ones we have. Our old feed URLs are being used, rather than the new redirected ones. But the redirect change just happened last week, so that's not too surprising.