Originally the captain was flying, and he was keeping up with MCAS using manual electrical trim. But near the end, he handed control to the first officer, so t

Originally the captain was flying, and he was keeping up with MCAS using manual electrical trim. But near the end, he handed control to the first officer, so that he could read the quick-reference handbook and troubleshoot. And the first officer, not knowing how much trim the captain had been using, failed to keep up with MCAS. Very sad - if the captain had kept flying, they may have survived.