Śrīmad-Bhāgavatam 10.37.5
Blended reading (Sanskrit + English inline)
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smayan
—
smiling
Kṛṣṇa smiles while inserting His arm into the horse's mouth. The smile is the signature of līlā — what looks like combat from outside is play from inside. Gauḍīyas treasure smayan-kṛṣṇa: the Lord who fights without losing His grin.
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bhujam-uttaram
—
His left arm
Uttara — northern, upper, left. Even the choice of arm is leisurely; the right is unneeded. The detail tells us Kṛṣṇa is not exerting.
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vaktre
—
in the mouth
The demon's weapon (the gaping mouth) becomes the entry-point of his death. Gauḍīyas read this as the standard līlā-grammar: the asura's chosen instrument is always what the Lord uses to undo him.
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bile
—
into a hole (in the ground)
The simile reduces Keśī to a small snake retreating into its burrow. The cosmic terror is sized down to a household nuisance the cowherd boy can handle one-handed.
When Kesi regained consciousness (labhda sanjna) he got up, and, opening his mouth, ran at Krsna. But Krsna just laughed. Showing his left thumb, Krsna said, "Do you want to eat something? Here , eat this." He then thrust his left arm into Kesi’s open mouth, in order to destroy his life, just as a snake easily enters a hole to kill a mouse.
The wicked demon (saḥ) got up. Or, though thrown at a distance, he (saḥ) got up. Just as a curious person will put a snake into a mouse hole without fear of mishap, Kṛṣṇa put his fist in Keśī’s mouth, which represented his life air.
Keśī wakes, charges again with mouth wide open — and Kṛṣṇa smiles and slides His left arm in, as casually as a child guides a small snake back into its hole. The demon's open jaws turn out to be the very doorway the Lord was looking for.
My note
Source: vedabase.io