They are like sticky keys. In my layout they're all on AltGr presses so you'd have to hold down the AltGr (RAlt) while pressing the dead key, then release both and type whatever you want the accent on. You can get the accent on both minuscles and majuscles this way (Åå or Ąą for instance). You can even make multiple accents for Vietnamese and suchlike by pressing more dead keys in sequence before releasing them with a letter: Thích Nhất Hạnh (notice that the ấ has both a circumflex and acute accents as required in Vietnamese). And you can "cheat" and put other stuff than accents there to make fancy compose sequences: In Linux/XKB you can use the circumflex to get superscripts like ¹²³⁴⁵ for instance (personally I'd like superscripts on the acute and subscripts on the grave). The dead key on AltGr+4 in Colemak[eD] is a Currency dead key for ¥₤ etc.

In the standard Colemak there's a dead key with lots of miscellaneous symbols on, but that didn't make it into the Linux implementation because those things aren't defined in the layout files but in the compose files and defining a new dead key would be considered a drastic change.

The other way in Linux is Compose, where you press the Compose key first and then enter two or more glyphs that form one of the many allowed compose sequences. Pressing Compose+a+' gets you á for instance. I've mapped my dead keys to harmonize with the Linux Compose sequences (apart from the quite common diaeresis/umlaut which deserves an unshifted position so I've used ; instead of " for it). That way it's easy to use a bit of each or whatever you prefer.

I'm so nerdy and I like it. ;)

I know ghen, you crafty bastard you. However, hijacking dead keys won't any time soon make it into a distro release, just saying. ;)

And honestly I don't thing the "special symbols" dead key will be a priority because it overlaps with the more popular Compose method and furthermore it's hard to agree on the details of it. Meanwhile we're free to modify the compose files on our own of course!