Jinul writes (Chinul's Works, p 141; in Collected Works of Korean Buddhism, vol 2):
At the moment of your death, wind and fire will oppress you, the four material elements (mahābhūta) will separate and scatter, and the mind will go mad, feeling stifled and cramped, and become subject to the inversions (viparyāsa) and distorted views. As you have no stratagem for soaring into the heavens above nor any plan through which to enter the earth below, you will cower in fright, bereft of everything on which you used to rely. Your physical body will be left behind as if it were a cicada’s cast-off shell. Confused about the road stretching before you, your lonely spirit will have to go on alone. Although you may have owned precious jewelry and priceless riches, you can take none of it with you. Although you may have relatives from prestigious households, ultimately not one of them can follow along behind to rescue you. This is what is meant by the statement, “What one makes oneself, one receives oneself; there is no one to take one’s place.”
Then quotes Baizhang (p 142),
"[At the time of your death,] all the unwholesome actions you performed throughout your lifetime will appear before you, either alarming or pleasing you. The six rebirth destinies (s.ad. gati) and the five aggregates of being (pañcaskandha) will appear before you, and you will see beautifully decorated houses, skiffs, carts, and palanquins all shining brilliantly. [These sights] make your mind dissolute so that the things you view with greed and lust are all transformed into pleasing sensory objects. You will be reborn at the spot where those sights are most intense, without one iota of choice in the matter; whether as a dragon or an ox, whether of high or low status, absolutely nothing is fixed."
Only enlightened beings are capable of choosing their birth. Jinul quotes Sengzhao (Straight Talk on the True Mind, in "Collected Works of Chinul", p 181):
"Saints abide in existence but are nonexistent; they dwell in nonexistence but are not nonexistent. Although they cling neither to existence nor nonexistence, they do not reject existence or nonexistence. Therefore, their light blends harmoniously with the troubles of the dusty world. They pass between the five destinies, calmly going, suddenly coming. Tranquil, they do nothing and yet there is nothing they do not do."
Therefore, ordinary people cannot do anything once dead to change their birth, and enlightened beings are free from the constraints of karma.
"If the Buddha-Nature is seen, there will be no seeing of a nature in any thing."
(Nirvana Sutra, T12n374p521b3; tr Yamamoto)
"Neither cultivation nor seated meditation — this is the pure Chan of Tathagata."
(Mazu Daoyi, X1321p3b23; tr. Jinhua Jia)
“Don’t rashly seek the true Buddha;
True Buddha can’t be found.
Does marvelous nature and spirit
Need tempering or refinement?
Mind is this mind carefree;
This face, the face at birth."
(Nanyue Mingzan: Enjoying the Way, tr. Jeff Shore; T2076p461b24-26)