Once students are signed into the site, they can respond to the question and then begin giving feedback to their classmates' responses by comparing two at a time and dragging/dropping the criteria on what they perceive as the better response. When the feedback period ends (this is set by the teacher), My Short Answer aggregates all the feedback and ranks the responses. Teachers can then show students which response they felt most met the criteria, which provides a wonderful opportunity for a class discussion about what made that response so strong.
Here are two great interactive slides that show what My Short Answer looks like from both the teacher and student experience.