Try to feel that your breath itself is silently making the sounds of
Hong and Sau.
It is quite clear that it is talking about the Hamsa Mantra also known as the Ajapa Japa which everyone does constantly but without any special effort.
There are plenty of excellent references from the Tantras on Hamsa but I'm quoting now from a minor Upanishad.
The Jeevatma (the soul of the physical individual) is under the
control of prana which goes up and down as well as apana. Apana pulls
Prana. Prana pulls Apana. He who knows and realizes this mutual pull
which is pulling to the top and bottom, understands yoga. 30 It goes
outside with the sound “ha” and goes again inside with the sound “sa”.
The beings keeping on chanting this mantra as “Hamsa”, “Hamsa”. 31 The
beings always keep on chanting this mantra day and night twenty one
thousand and one hundred times. 32 This mantra which is called “Ajapa
Gayatri” would give salvation to all yogis. Just a thought of this
mantra, would help one get rid of all sins. 33 There are no practices
as holy as this, no chanting which is equivalent to this, and no
wisdom equivalent to this and in future also this is not likely to be
there. 34 This Ajapa Gayatri which rises from the Kundalani supports
the soul. This is the greatest among the sciences of the soul. He who
knows this will know the Vedas.
From the Yoga Chudamani Upanishad linked with the Samaveda.
Ham is Ha+anuswara (or the Bindu) which that website has written as Hang or Ha+ng. So, instead of writing the Bindu as "m" they have used "ng" (Bengalis in particular often do that).
To write Sauh instead of Sa is not wrong too. Sauh is the Parashakti Bija where as Ham (or Haum) is the Shiva Bija. When these two Bijas are combined the Hamsa Mantra is formed.