A KAR A ME TU TI SU NAG PO SHI SHI MAL MAL SO HA

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This mantra is associated with the Nirmanakaya, or physically manifest body of the Buddha, and is known as the purification mantra, or the mantra that offers protection from negative energies. The mantra can be broken into four groups of seed syllables:

A KAR A ME is the antidote.
TU TI SU NAG PO represents the negative emotions that are purified.
SHI SHI MAL MAL is the benefit or result of the mantra.
SO HA cuts doubts, misconceptions and wrong views.

In greater detail, each of the syllables has the following meaning:
A represents space and Sherap Chamma, the loving mother.
KAR represents the clarity of Shenla Odkar.
A represents method and Sangpo Bumtri, one of the primordial Buddhas
ME represents wisdom and Tonpa Shenrab.
(The above four syllables make up the antidote deities and the antidote wisdoms.)
TU represents the afflictions of the hell realm.
TI represents the afflictions of the hungry ghost realm.
SU represents the afflictions of the animal realm.
NAG PO represents the afflictions of the three higher realms of the humans, demi-gods and gods.
(Above are the four afflictions that are purified by the four wisdoms and four deities of the first four syllables.)
SHI SHI overcomes the afflictions.
MAL MAL represents the bliss that arises once the afflictions are dissolved.
SO HA represents the realization of union, the inseparable state.

If you are repeating this mantra rapidly you can recite the SO HA every hundredth repetition, but there are many variations depending on the situation. If you are using the mantra for purification you need not recite SO HA. But if you are trying to cut through a negative influence you would always use SO AH at the end of each repetition.