there is a small tab that sticks up at the back in the centre, this tab moves up between the black U shape and blocks the optical sensor, once at the top the bed will need levelling down (tighten the adjusting screws) dont even bother with the inbuilt calibration- its an absolute pain, but set the print to start, once the bed is hot and the nozzle starts its moving turn off the printer at the power switch, then while its at temperature, manually move the print head over the glass by the three screws, and adjust until a piece of paper on the bed will just slip out from under the head, then repeat for all the screws, once done recheck the first screw, the thickness of the paper should leave just enough clearance for the first layer. if the nozzle clicks it is generally too close so add a small amount of tightening to that area of the bed to improve the clearance.