It is not a meditation to open those chakra. It is a chariot meditation that creates the path of the kundalini to rise.
Not all Ss are the same in Sanskrit. There are several sibilants. All sibilants are serpent letters, so they relate to the kundalini energy. Each of them embody a different element, depending on what type of sound they make:
:arrow: śa as in Śiva (mispronounced Shiva in English) and Śākti (mispronounced Shakti in English) is a palatal sound, which means the tongue touches the palate when pronouncing/vibrating it. Palatal sounds derive from the vowel "I" and manifest through the air element;
:arrow: ṣa is a cerebral sound, which means the tongue touches a bit further back toward the throat when pronouncing it/vibrating it. Cerebral sounds derive from the ṛ-vowel, not to confuse with the semivowel 'R'. They manifest through the the fire element.
:arrow: sa is a dental sound, which means the touch is placed in the palate slightly behind the front teeth when pronouncing it/vibrating it. Palatal sounds derive from the ḷ-vowel, again not to confuse with the semivowel 'L'. They manifest through the earth element.
They are all mahaprana sounds, which means they are aspirated, which is why every time there is an S in Satanic kabbalah JoS instructions are to hiss them. I am mentioning Sanskrit because it's the foundation of mantra in SS Kabbalah.
In this case, I'm assuming this is the fire variant, and the reasons why fire is used are simple:
:arrow: it stimulates the kundalini, which lies at the base of your Sushumna, with its mouth biting the muladhara chakra
:arrow: the position of the kundalini is the reason why the muladhara chakra feels immensely hot when activate
:arrow: fire is a cardinal element, meaning it initiates movement. Specifically, movement upward aka ascending movement
:arrow: chariot meditations refer to the Chariot of the Sun. Sun is Sowilo, Sammas in Akkadian.
The Chariot of the Sun is carried from dawn (the lowest point of the horizon aka the muladhara chakra) to noon (the highest point in the horizon, aka the heavens, aka the sahasrara chakra) and to dusk (back to the lowest point of the point of the horizons but from the other side, still the muladhara's domain. This is what the mantra pattern going first up, then down is trying to emulate.
The 'heart' chakra is no longer called heart chakra. This is because the real spiritual heart is in the head, and it's the pineal gland. The opening meditations have been updated to reflect this newfound awareness, and the rest of the website will most likely follow suit. It is now called middle/fourth chakra. It is also not a mix of fire and air. If anything, it is clear it is a mix of air and akasha, because aither keeps our soul together, which is similar to what the anahata chakra does.
Based on that, the A there is justified. The A is the mother of all guttural sounds, which manifests through the element of akasha like you stated.
M is a labial sound, whose mother is the vowel 'U', which manifests through the water element. The water in relation to the sahasrara chakra is more about it the concept of 'heavenly waters' rather than the element of water per se, which is in the svadhisthana chakra.
With this meditation you are not working to open the chakra or working specifically with any element. You are simply preparing the path for the kundalini to raise to her throne and the middle chakra acts as a connector to go from point A to point B, keeping S and M in balance. More than that, it also harmonises the energies, creating a balanced path, as the vowel relate to harmony between solar (S) and lunar (M) energies, masculine and feminine. You are also reawakening proper energy flow that is needed for spiritual beings at our stage and beyond. You are purifying your soul, and also achieving many other things. This is the beauty of Chariot meditations.
