This little yet cleverly designed piece, slips in the lower edge of the left vertical carriage to make it meet the end stop switch to bring a few millimeters extra of clearance.
This solves the problem some CR-10S and upgraded CR-10 have: when the Z axe slides the X beam downwards, the right carriage lower edge ends up resting against the second motor support just a tad before the left carriage lower edge triggers the Z end switch, thus slightly tilting the X beam, such that it loses its horizon in part of its Z axis trajectory. This tiny piece triggers the switch at an higher height, preventing the unwanted contact in the right side.
To install, slide the notch under the carriage plate, right above the switch. Notice the shape, trimmed to prevent the contact against the vertical post, and ensure the matching of this trimming. You may need to add some blue tape inside the notch to make it fit more tightly.
Edition: You may need to add some washers under the bed's springs to keep the tension needed to level it. [Thanks to Cymen to remind me of this detail in the comments].