As the head continues backward, the probe tip falls off the back edge of the sample ridge. The cantilever straightens out, moving the reflected laser beam to the front of the photodetector.
The feedback loops are again called upon as they now rotate the tripod assembly downward by just the right amount to restore enough bend in the cantilever to move the laser back to the center of the photodetector.
The AFM records the these ultrafine tipping motions (corresponding to sudden increases or decreases in the top electromagnet current). By means of software calibration tables, these currents can then be translated into measurements of actual height changes along the sample surface.
