1. A computer-implemented method of triggering a detector to detect a moving feature within a video stream, the method comprising: defining a static measure indicative of the presence of a moving feature of interest within a frame of the video stream at a computer; repeatedly determining a value of said static measure for a sequence of frames to generate a corresponding sequence of values at said computer, wherein said sequence of values indicate a head turning sequence within said sequence of frames, and wherein said sequence of values provides a measure of a symmetry of said moving feature of interest; and triggering a face detector configured to detect said moving feature within said video stream only in dependence upon a characteristic of the sequence of values at said computer, wherein said characteristic is a local extremum, wherein said local extremum is a peak measure of said symmetry of said moving feature of interest, and signifies a frame containing a full frontal face, thereby conserving resources by triggering said face detector to run only when it is most likely to be successful.