What does it mean, 'inner' and 'outer trigram' (popup)
Duì below Gèn: with Duì inside, you or the situation is social, you are open. But towards the outside there is holding back, staying aloof, because it is important to stay true to one's own values in the middle of others. That is possible by staying master over one's emotions - or by being indifferent to others, with a solid cover to protect your heart. Two sides, like every hexagram and every line has two sides. It is always a choice which side one takes, the positive or the negative one.
Ideogram of the hexagram name: At left one hand, maybe two, and at right a cauldron with a circle, meaning round or empty. In all ages, long before any arithmetic, the circle indicated already 'empty', hollow, nothing, zero. The hand is a special form, meaning to clasp one's hands and to bow with reverence.
But of course empty has two sides: the space where one can receive - or the lack of substance. SUN: decrease; damage; harm; lose, be mean/shabby, be sarcastic/caustic.
The 'Great Image' says: at the foot of the mountain is a lake: decrease. The noble one controls anger and restrains passion.
Hex.41 is the contrast of 31, which is about completing and embracing others.
Hex.41 is about the basics of oneself, your fundament, stripped from everything not-you.