They aim for maximal stability of direction and flux. "It is essential that the direction of the neutrino beam be stable to within 1/20 th of a degree, and tha

They aim for maximal stability of direction and flux. "It is essential that the direction of the neutrino beam be stable to within 1/20 th of a degree, and that the intensity of the beam be constant over time ..." http://t2k-experiment.org/t2k/ http://t2k-experiment.org/t2k/ While turning it on and off or steering it differently seems like it would help generate correlations with the detectors to distinguish T2K daughter products from those of other phenomena, the total set of detections are fleetingly rare. Moreover, the background is reasonably well characterized from the more than ten years before T2K existed, peppered with data from the various periods when T2K's beam was off for extended periods. A brief explanation of why the T2K's beam is angled slightly away from the SK detectors is elsewhere at the same link.