Last back is a term you'll often hear in between points watching a game of ultimate. Basically, what it signifies is that someone on the sideline wants to get into the game. The team which just got scored on has to run back to the other end zone to recieve the pull. It the person who called last back gets to the endzone before the team which just got scored on then the last peron who just played the last point to get to the endzone has to sit out the next point.