This is a Pentax flash system feature where a cable-connected flash fires in combination with the camera's built-in flash. The built-in flash provides one third of the required light and the off-camera flash provides the rest. Pentax and Metz supports this on their more advanced models, and Sigma on all models. This mode does not appear to be meaningful when using wireless P-TTL. In any case, setting flash-based exposure compensation can be used instead, for models which support that.
Contrast Control Sync