There's a flash of neutrinos as the supernova core collapses and it precedes the light by a bit. One reason being the neutrinos don't really care there's a whole supernova in the way but EM radiation does.

https://en.wikipedia.org/wiki/Supernova_neutrinos

Supernovas emit lots of different kinds of particles and radiation.

Neutrinos depart & arrive before any other radiation. The gap is on the order of hours. So neutrinos can function as an early warning system.

There's a "How It Works" page on their (very small, minimal) site:

https://www.exaluminal.com/#work