dip the flag —(of a ship) give a salute to another ship by making the flag go down for a moment and then up again:
- Today, “dipping the flag” is an act of courtesy; men-of-war do not do it to one another.
drop the flag—
1. (Sport) indicate the start or finish of a race using a flag:
- When the first horse reaches the winning post, one of the judges there drops a flag.
2. give the signal for the start or finish of smth:
- It would be a great help to the leader of a future expedition in making up his mind when to “drop the flag.”