"Both glides were filmed by sticking a – relatively cheap – digital camera out of the window of a train as it arrived at a station. The ‘trick’ is the camera collects images at a rate of 210 per second – but the film is played back at 30 frames per second. So, every seven seconds of footage that you watch corresponds to 1 real second. At least at the start, one real second is plenty of time for someone to move into, then out of, the camera’s field of view, but isn’t enough time for them to really do much: hence, the frozen effect. It breaks down towards the end not because I’m doing something clever with the frame rates (captured or replayed), but simply because the train was stopping!"