The detector is made of thousands of light sensors arranged in a a km-scale 3D grid. When a neutrino interaction causes a flash of light inside this volume, multiple sensors detect the photons with nanosecond-level time resolution. So a 3D map can be made of where the light started and how it moved over time.
For a macroscale analogue, imagine a large 3D grid of microphones all recording sound. If you fired a cannon from inside this grid and looked at the waveforms from all the microphones, you could work out where the cannon was, and also form a pretty good guess of what direction it was pointed.
For a macroscale analogue, imagine a large 3D grid of microphones all recording sound. If you fired a cannon from inside this grid and looked at the waveforms from all the microphones, you could work out where the cannon was, and also form a pretty good guess of what direction it was pointed.