They’d teach them how to stand.
It’s not that they’d teach them a variety of stances, no. They’d actually re-teach them how to stand, how to spread their weight, where their feet are supposed to go, and how best to balance themselves. They’ll usually spend the entire first lesson teaching that with a lot of modifying the body’s position to hold it. This is practiced on the first day and, sometimes, the second until it’s second nature.
The reason for this is: base.
All students need a solid foundation and it begins with your feet, your body, and how you hold yourself. Your base is the first and most important skill you’ll ever learn in any martial arts program, it informs every skill you are about to acquire. It’s important to remember that nothing about martial combat that is “native” or “natural”. It’s a learned skill.
Your foundation is necessary to be able to generate power and to take hits. It’s about being able to properly balance in order to do the techniques. To keep your back upright, your gut tight, your shoulders spread, to breathe from the diaphram, to bend the knees slightly, and lean forward onto the balls of your feet rather than leaving all your weight in your heels. Training in this is central to a student’s ability to perform well and to survive. Without an understanding of balance or of the body before strikes, without understanding the pieces which make up a greater action.
It’s actually up to the individual teacher as to whether or not they’ll explain why this is to their students. I suggest you do because most audiences genuinely don’t know.
Every style has a different way to stand and every school has a different way of teaching it. Some spend more time on it than others, some less so than others. Sometimes they build it in with learning to fall/roll, where one slaps the ground to spread the force of the fall.
It’s a very “hands on” experience for the students. The teacher will spend a fair amount of time readjusting their body. Prodding the spine so they open up the chest, nudging their feet wider apart, resetting the hips, and poking the stomach. Later in training, they may bring a tool like a light stick to test their ability with a few smacks but that won’t be on the first day or the second.
I cannot express how important this actually is and how often it gets skipped, because it’s either considered to be uninteresting or the author genuinely doesn’t know.