controller functions that are only accessible to jquery or ajax

[eluser]xeroblast[/eluser]
as i was reading this thread : http://ellislab.com/forums/viewthread/229428/ .. a question pops in my mind..

if making function in controller to be private and not be accessible in the URL. is there a way to send data via ajax and its URL is not accessible by the browser but only ajax?

i tried making a private function but ajax cant send the data to the URL i give.