29. A nontransitory computer-readable medium having instructions stored thereon that, upon execution by one or more processors of a wearable head-mounted display (HMD), cause the wearable HMD to carry out operations comprising: operating in a background detection state; while operating in the background detection state, carrying out functions of the background state including, measuring three orthogonal components of a background magnetic field using three orthogonal measurement axes of magnetometer device of the wearable HMD, and determining a field magnitude of the background magnetic field from the three measured orthogonal components, determining an occurrence of a trigger from a hand-wearable magnet (HWM) at a time T start upon detecting a perturbation by the HWM of the determined field magnitude at least as large as a perturbation threshold, and upon determining the occurrence of the trigger, transitioning to operating in a gesture detection state; and while operating in the gesture detection state, carrying out functions of the gesture detection state including, for the duration of the gesture detection state, tracking motion of the HWM by determining time derivatives of magnetic field strength measured by the magnetometer device along each of the three orthogonal measurement axes, making a comparison of the determined time derivatives with one or more sets of pre-determined time derivatives of magnetic field strength, wherein each of the one or more sets is configured to be stored at the wearable HMD and each is associated with a respective known gesture, upon matching the determined time derivatives with a particular set of the one or more sets based on the comparison, identifying the respective known gesture associated with the particular set, and transitioning to operating in the background detection state upon both of, measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold, and determining an expiration of a time interval W that begins at T start .
31. The nontransitory computer-readable medium of claim 29 , wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains at least as large as the perturbation threshold for at least a first threshold length of time, and wherein measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains less than the perturbation threshold for at least a second threshold length of time.