The Two Parts Of Every Command
There are two parts to every command. They are: (one) the preparation and (two) the execution.
The preparation tells the band what they are about to do. The execution tells them to do it. For example...
"Band...Atten (preparation) Hut!" (execution)
Both parts of every command must be done to complete a proper command. The Band/Corps must know what they are supposed to do and when they should do it.
Since whistle commands evolved from oral commands, they also have the same two parts. Usually the preparation whistle is done while the drum major does some type of signal with a baton or mace. The execution whistle is a short whistle, or series of short whistles that tells the band to do the command.
Make sure that every command is done clearly and has both a preparation and execution. This will keep your band/corps executing your command crisply and will eliminate confusion in your directions.
(Get more information about using a whistle by also reading "The Classic Drum Major Whistle.")