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We investigate how smartphones can be used to mediate the manipulation of smartphone-based content in spatial augmented reality (SAR). A major challenge here is in seamlessly transitioning a phone between its use as a smartphone to its use as a controller for SAR. Most users are familiar with hand extension as a way for using a remote control for SAR. We therefore propose to use hand extension as an intuitive mode switching mechanism for switching back and forth between the mobile interaction mode and the spatial interaction mode. Based on this intuitive mode switch, our technique enables the user to push smartphone content to an external SAR environment, interact with the external content, rotate-scale-translate it, and pull the content back into the smartphone, all the while ensuring no conflict between mobile interaction and spatial interaction. To ensure feasibility of hand extension as mode switch, we evaluate the classification of extended and retracted states of the smartphone based on the phone’s relative 3D position with respect to the user’s head while varying user postures, surface distances, and target locations. Our results show that a random forest classifier can classify the extended and retracted states with a 96% accuracy on average.