Fig 3 here represent an extreme example of tucking (when your tailbone is in front of your waist. What is the purpose of tucking? As we can see here, when the tailbone is too far ahead or behind relative to the waist, then it's a back bend, where the waist's ability to twist/rotate from side to side is severely limited. The best position for rotation to happen is when the upper body is in the neutral position (Fig 1, 4).
So why do the masters say tucking then? I personally feel the instructions (eg. tuck the coccyx and hollow you chest...) would be much clearer if they phrase them as "don't do this (eg. don't stick your butt out, don't stick your chest out). The reason we have these instruction is that remember that internal martial art is a fairly recent phenomenon. Before that it's all external martial art, where it's okay to sticking the butt and chest out is quite common. When internal martial art came along, masters say 'don't do this or that, I know that works for external martial art style. But our skills are totally different, and they're based on a different set of body mechanics, so don't do what they do..."
No matter who you are, if the requirement is knees don't pass the toes, then the only way your head can be directly above your feet is if you stick out your chest forward and the butt out backward.
If you don't want to do this back bend, then your knees has to go pass the toes:
Or, you have to go pass parallel, let your butt sink below the knees:
Or, really open your legs, point your toes sideways instead of straight ahead, like you're doing a sumo squat:
Because very few people have this type of knee, ankle strength and flexibility:
And/or you take a bigger stance. Taking a bigger stance means you can't support all your weight on just one leg. But given how the human body is built, if you want to go low, you have to do make a compromise somewhere.
If tucking is emphasized more in Chen Style, it's because the stances are usually bigger and lower during the practice. The body will naturally try to cheat by sticking out the butt. The masters want to make sure the student's waist is neutral and free, which taiji quan skills depend on.