You actually analyzed the language! Was the movie that bad? Not Баллада о солдате, I hope. In any case, I'm impressed!
Concerning your question:
Imagine that you're a Russian drill sergeant yelling out instructions to some new recruits - these Gleb Gleboviches can't stand at attention, much less march anywhere in a formation.
You go up to one yokel and yell in his face, "Стой прямо!" Sounds good and authoritative! The kid shakes in his boots and wets himself. Now you go up to a group of three bumpkins and yell, "Стойте прямо!" What's wrong with this picture? Doesn't really sound all that good or authoritative, does it? Sounds like you're yelling at a ballet class or a bunch of gymnasts. Why? You've used a plural imperative - as you should since you're addressing a group of raw youths. The trouble is that the plural imperative is also the "polite" - вы - form. The last thing a drill sergeant like you wants to do on the first day of boot camp is appear polite!
But thanks to the genius and wealth of the Russian language, you have another choice: the infinitive! "Всем быть на местах! Стоять прямо! В строю не разговаривать! Молчать!"
"Line up! Stand straight! No talking in the ranks! Shut up!"
Boy, those hayseeds are sweating bullets now! You're in total control. Now you can ask them where they're from and go into your "There's only TWO things that come from Nizhni Novgorod! Steers and..." routine. * * *
The point is that the infinitive can function as an imperative to express strong or strict commands - and not just in a military context.
For example, many "prohibitory" signs in Russian use an infinitive imperative:Не курить! "No smoking!"
По траве не ходить! "Stay off the grass!"
Also simple everyday commands: Сидеть тихо, мирно! "Sit quietly and still!"
Вам начинать! "You start."
(Or in games like chess, "You open."
Or "Your move first.")
Finally, remember the movie The Russians are coming, the Russians are coming? (I know I'm dating myself, but this whole thing started off with YOU watching an old Russian war movie.) A Russian sub runs aground in a New England port village. None of the Russians onboard speak English, so they work out a few basic phrases using some tour book. They go ashore and soon we see a Russian sailor running through the village, reading a piece of paper and yelling, "Emergency! Emergency! Everyone for to get from street!"
What was the original Russian for "for to get from street"? Judging from the English translation, it was an infinitive used as an imperative: убраться с улицы!
For to have a good day!