problem with understanding Texture2D.ReadPixels

hi, i have a little problem to understand Texture2D.ReadPixels function. basically i understand the part of the Rect that i need to read but other part with DestX and DestY confuses me and i am not able to render the texture of the read pixels where i want on the screen. check out the attachment i made. alt text

here is the code i used:

// Create a texture the size of the screen, RGB24 format
 var width = 250;
 var height = 600;
 var tex = new Texture2D( width, height, TextureFormat.RGB24, false );
 // Read screen contents into the texture
 tex.ReadPixels(Rect(5,250, 250, 600), 10, 10 );

Where you display it on the screen depends on what you're using. (A quad? GUITexture? OnGUI?) The DestX/Y parameters don't have anything to do with that.