The crucial point is that "yo sé" couldn't be shortened to "no", in the same way...

jeorgun on April 8, 2015 | parent | context | favorite | on: What part of “No, Totally” don't you understand?

The crucial point is that "yo sé" couldn't be shortened to "no", in the same way that "I know" can be shortened to "no" (via "I know" -> "know" -> "no").