There's some argument about the origin of the twelfth parts (Dodekatamoiria in Hellenistic Astrology). with at least one writer claiming to detect a reference in Babylonian times but it was Hellenistic Astrology that really made use of them, a techique passed on to Jyotish).
Firstly, what matters is the planets rather than the signs -The twelfth part of a planet refers to a place of importance for a planet. The Dodekatamoiria are linked to the Sun/Moon cycle of 12/13 lunations in a year. Though they later became standardised as twelfths of a sign.
Firmicus Maternus says that Duodecatemoria (latin version) are seen by many astrologers as revealing the secrets hidden in the chart. He calculates them by multiplying the degree that a planet is placed in by 12 and then measuring that number of degrees from the sign in which the planet is placed.
His example goes as follows. Sun at 5 degrees 5 minutes Aries. Multiply this by 12 to get a total of 61 degrees. Move 61 degrees from the beginning of Aries and you come to the 1st degree of Gemini You would examine the first degree of Gemini in terms of which Bounds it lies in which is the Bounds of Mercury. That's not to bad, Mercury is rather neutral but the bounds of Saturn or Mars might indicate that the affairs of the Sun will go badly, the bounds of Venus or Jupiter might indicate that all will be well.
Rhetorius the Egyptian seems to interpret the Dodecatemorion (sic) as beneficial if they trine or sextile their planet but adverse if the are in opposition of square to it. So the interpretation seems to depend on whether the twelfth part makes a beneficial or a harmful aspect and also in which bounds the twelfth part falls. The signs may give more information, for example if the Moon's twelfth part forms a harmful aspect in a quadrapedal sign (such as Aries or Leo), the native's mother may come to harm from wild beasts. If it is from a human sign (such as Aquarius or Virgo) the harm may come from other people, such as muggers.