The goalkeepers pass the ball to their teammate inside the center rectangles (1); the coach will decide which team is the attacking team and which is the defening one; blue team is the attacking here.
The first receiver of the defending team must dribble outside the center rectangle and score too before he can run back to help his teammates (3 vs 3+1) who are defending outnumbered 2 vs 3+1 meanwhile (2).
The attacking team can score a goal only the free player has played the ball as well, as quick as possible before the third defender can run back (3).
If defending team player misses the first shoot he cannot run back, so his teammates will defend outnumbered (2 vs 3+1) till the end of the move.
See the highlighted arrow.