The gantry moves around on the cables held by the pylons on the edges. If it were a transmitter, the beam of radio waves would shoot out in a different direction based on what part of the surface it was illuminating. However, the really clever thing about FAST is that the surface is not just a passive metal net like Arecibo, but an active surface of triangular panels. The panels reform the normally spherical surface into a parabola. So you can imagine the surface deforming as the gantry shuttles overhead to make the correct optical surface. This is pretty hard to do right, so most observations so far have been setting the surface to look straight up and leaving the gantry in place, observing a stripe as the sky passes overhead.