why does this work?

[digitalecartoon]

I know that this works:

clip.onPress = clip.onRelease = function () {
	trace("yeah");
};

(both when pressed and released a trace occurs)
But I can't find anything about it in the help file WHY this is correct and why it works. Can someone lead me to the page where it says this is correct syntax for combining multiple event handlers into one? I can only find this one:
on (press, release) ...