radar guns...how they work?

ok...so i'm curious into the details of how a radar gun works in teh game of baseball and was hoping somebody will know....during baseball games they put up the speed of the pitch...now is this the speed of the ball just as it left the hand of the pitcher? is it the speed of the pitch right before reaching the plate? is it an average speed of the ball over the entire 60 ft?....

the reason i ask this is becuase i dont think it can be teh speed of the ball out of hand otherwise 2 and 4 seam fastballs would be the same, as i believe would be changeups too...because it is the difference in spin tha tcuases these pitches to be different speedsx....and also balls slow down as they approach the plate..so is it the last speed right before the catcher catches it that the radar gun records or an avg????? does anyone know???