Space.com: Warm pasta helps hot, angry neutron stars cool down

Neutron stars, as the name implies, have plenty of neutrons, all zipping around at high energies with lots of momentum. But the neutrino recipe requires producing a low-energy proton with almost no momentum. Momentum can't just disappear though. It's always conserved. That's Isaac Newton's First Law of Motion. (It's also why if your car stops suddenly and you're not wearing a seatbelt you go flying out the window.)