How does push to talk work?

So I've always liked those 'push to talk' phones (like Nextel has). I mean, its really cool to be able to just push a button and talk to someone, walkie-talkie style.

But I've been curious as to how they actually work from a technical perspective? The wikipedia page makes it sound like its a seperate connection protocol of some sort (e.g. not a normal cellular call), but it isn't exactly clear.

Does anybody know?

I had hoped to get a Casio G'Zone Boulder this month as a review unit but it doesn't look like that may happen.  I haven't used a PTT phone yet myself and wanted to try it out.  Of course I don't have any friends with PTT, so that kind of defeats the purpose of paying for the service :)