The ZWO M68 Off-Axis Guider (OAG) is designed for full-frame and APS-C cameras.
The OAG picks off light coming into the optical tube which is outside of the light path to the imaging camera. It has a small mirror that deflects the light to a guide camera. In this case, the guide camera is a ZWO ASI290MM-Mini (monochrome). The guiding image is continuously processed by PHD2 software in short exposures which is tracking on a guide star, compensating for any tracking error in the telescope mount.