Shizentai means "natural posture". However, it is not your ordinary posture.
You must learn the natural posture in order to stand naturally. Yes. Very complicated but you will get used to the posture.
The key point are;
It does not sound like a natural posture. Very uncomfortable for the first time you try.
Probably you know there is a difference between being relaxing and being lazy.
We do not want to be lazy. We need an effort to keep your posture but you can be relax.
That is what we want. Ok, this is how we create this natural posture.
1. lift your shoulders
2. bring your shoulders back (it feels like your shoulder blades meet at the back)
3. drop your shoulders.
This naturally sticks your chest out so that your shoulders will not roll
4. keep your chin in but not too much.
5. keep the back of your neck straight.
1. tighten your buttocks
2. heels together and toes apart (45 degrees between toes)
When you tighten your buttocks, probably you feel the abs and the hamstrings
You feel taller.
Now I know it feels weird but you will get used to it. In other words, do it until it becomes "natural".