I don't know how the use of platz came about. Knowing more than a few words in German, I was a bit confused myself.
Maybe because niederschlagen doesn't exactly roll off the tongue for a dog command. It also has too many syllables.
To the dog, it doesn't matter. She just associates a word with an action. You could say 'chili' and teach her down.
Fuß is of course, foot as in Fußball and you want the dog at your foot when you give the command.
Bear, Anna, Molly, Ossie, and The Countess Lisl von Schlaf
An Unhappy German is a Sour Kraut!