The trigrams of a hexagram are called the lower or inner trigram, and the upper or outer trigram.
In Chinese they bear the names ZHEN1 (divination) and HUI4 (repentance).
Zhen, "to divine", is a cauldron, a Ding, the same one as the name of hexagram 50, with (later) at the top a 'pu' added. Pu is the crack in the oracle-bone, the diviner, the act of divination or its answer, to predict. It is a drawing of the crack, caused by the heat.
Hui, "repent", is a picture of a mother (a woman with breasts) and something on her head, like branches coming out of it, or according to Wang Hongyuan a feather headdress (mei3). She is the teacher, the one who says no, most of the characters with this mother in it have meanings like regret, trouble, to improve, to correct, dark and such. Mei means on oracle bones: clouded sky, dark weather. Later: always, every, fertile, rich, plural. Mei3: beautiful, every, frequently; often; whenever (with heart: repent, the monkey mind...). With the radical words or speak: plan (of the ancients, which has to be done), warning, teach.
But as name for the upper trigram it's radical is pu, the same pu as in zhen, the sign of divination or the diviner.
Together they give a clue about the meaning of the trigrams. The inner or lower one is more or less predestined, ordained by the gods, something one has to do or follow. One can consult the gods and act according to their advice, and then things may turn out auspicious. It is probably also close to instinctive behavior and inescapable reactions. The inborn wisdom, instinct, intuition.
The upper or outer one is, to some extent, one's own choice and can be influenced by one's thinking, wishes or conscious reactions. One's own feelings can change the course of events: the 'time'. Many actions will cause regret, but regret is also the beginning of change for the better, and the surroundings will react to one's actions.
The peculiar thing about HUI in 'regret disappears' is, that it figures like this only in the hexagrams from 31 to 64, but in more places than the other uses of regret.
The character ZHEN1 can be found in the Far-East dictionary nr.5749, Mathews’ nr.346, Karlgren GSR.nr.834, Li Leyi p.465, Schuessler p.822, Wieger p.150, Wang HongYuan p.184, Ricci 609, rad.154/2. Mei3,4 : Ricci 7673, Wang Hongyuan 180, Li Leyi 214, Far East 2727 The character HUI3 can be found in the Far-East dictionary nr.1584, Mathews’ nr.2335/6/7, Karlgren GSR.nr.947, Schuessler p.257, Ricci 5067, ?, rad.61/6.