We show ExtensionClip, a system that offers back-of-device touch interaction for a cardboard head-mounted display; it links the back of the smartphone to its touchscreen to greatly enhance the touch input area and usability. ExtensionClip sandwiches the smartphone between two magnets; the front magnet, which tracks the position of the back magnet, triggers touch events on the smartphone’s touchscreen by capacitive coupling. ExtensionClip detects the user’s touch of the back magnet from a change in the size of the touch area on the smartphone’s touchscreen; thus ExtensionClip offers pointing functionality like a mouse. We fabricate an ExtensionClip prototype and conduct user studies. In the first study, we determine the threshold needed to detect the user’s touch on the back magnet. Next, we measure the precision and the selection speed possible with ExtensionClip.