Mantra

I do not know how to use mantra myself, but from what I heard I imagine that mantra is the ability to listen for the various sounds that a person's body makes. I heard that they can predict a person's next move by listening. With training the range of Mantra can be increased.

Mantra (心綱（マントラ）Mantora, Kanji means "Mind Rope") is an ability some Sky Islands natives are born with. Whether it can be learned is debatable. Gan Fall says that it can be trained, implying that it is a learnable skill. However, Aisa was born with it and is unable to control it at all. This implies that it is an inherent power, and that only people born with it can train it for improvement. It allows the user to detect people at a distance without the other person knowing they are there. This makes it an excellent skill for planning battles. A fully trained user can predict a person's movements as well as their feelings. However, a user with this ability cannot detect a person inside another living thing or something that is not being mentally controlled. Luffy used this to his advantage against Enel by using ricochet from his punches to strike Enel.

It first appeared in Chapter 246 and Episode 160, when Satori used it when Sanji, Luffy and Usopp went through the Ordeal of Balls. Aisa is the only known Shandian who has this ability. It served as a plot point when she helps Luffy locate Enel. In the past, it was suggested that one of the Shandians priest had previously carried the ability. It was a dying priest who said he heard "the voice of god". "The voice of god" told him to "Spill blood on the altar" to kill the disease, meaning to sacrifice their most beautiful woman. However, this could just be delirium caused by the Tree Fever he had contracted.

The mantra in each priest and Enel was displayed in a different fashion. Enel's mantra was the strongest as the ability was enhanced when combined with his Devil Fruit, the Goro Goro no Mi. Enel explained that he could pick up electromagnetic waves allowing him to overhear conversations and everything around him having a range of, roughly, the size of Skypiea. Satori was the weakest. This was proven when he could not tell Luffy was behind him. Satori's brothers Hotori and Kotori lacked any form of mantra.

It is unknown whether Mantra is in any way related to the ability of Haki or in fact the same ability. Boa Hancock's sisters, Boa Marigold and Boa Sandersonia displayed the ability to use Haki in a way that appeared similar to Mantra.

During the near end of the war between the Marines and the Whitebeard Pirates, Coby displayed an ability similar to Mantra, but it was said that it is a certain type of Haki.

After the war when Rayleigh was explaining the different types of Haki to Luffy he confirmed that Mantra is a type of Haki called the "Color of Observation"

Trivia

 * The concept of Mantra is loosely based the style of thinking in the Hindu, of which it also goes by the same name.