It's basically where you try and kill sans but he survived and he blocks your attacks if phase 2, then you try and kill him again and your attack does no affect, then gaster comes to help sans, sans skull cracks like gasters, sans eyes glitch and he does crazy attacks and flips the box upside down and changes it shape, then sans fails and he dies, then gaster kills you. In some versions sans has more than 3 phases and he battles you forever, he even has a phase 1000 in some versions.