Hate to say this, but Bungie, we already knew what the X button did in Halo 3. But, in case you were one of the many non-interested Halo parties not paying attention, the Halo folks have now clearly defined it for everyone. From Bungie:
You guys did see the final revelation of the mysterious X-Button. It’s what’s known (at the moment) as Equipment. The two very plain examples shown in the Vidoc were the Bubble Shield and the “Tripmine.” Both can be used in all manner of strategic and hilarious ways. There might have been another, but you can wait a month to see that one.
There you go. Feel better now?