Without the shift functions you need to move the projector around to get it to match up with the screen. With shift you get to move a little joystick around on the front (or top!) of the box that moves a set of mirrors and/or prisms in the light path that shift the image so that your PJ doesn't...
Welcome! I just put my 120" screen in and am looking at the 3010's replacement, the 3020, or more likely it's big brother, the 5020. Pretty sure I need the shift capability on the 50!