how does non-rolling admissions work?

I know what non-rolling is - they tell everyone at the same time. My question is, do they make the decisions all at the same time as well?

I think that's unlikely for 2 reasons:

1. the committee doesn't have time to discuss 800 ppl at the same time in march.

2. the interviewers might not even remember what the applicant was like..

So does anyone know how exactly the non-rolling admissions work? they must be making decisions along the way but just choose to tell us at the last minute?