S. R. Faruqi:

On the theme of the beloved's lips as being called rubies and rose-leaves, see

{20,2},

where the beauty of the lips has been expressed with an innocent amazement. In the present verse there's a tricky 'innocence', from which Ibn-e Insha too has benefited:

kal chaudhavīñ kī rāt thī shab bhar rahā charchā tirā
kuchh ne kahā vuh chāñd hai kuchh ne kahā chahrā tirā

[last night was the night of the fourteenth-- all night there was mention of you
some said that was the moon, some said your face]

The difference is that in Mir's verse there's more arrangement for wordplay. The wordplay between kahe in the first line and bāt in the second line; and then in the second line itself between bāt and hoñṭ hilā ; and then for the thing under discussion (the lips of the beloved) themselves to be given authority to decide ('you just move your lips')-- all these things provide extremely enjoyable wordplay.

Then, in moving the lips the point is that whether they are rubies or rose-leaves, they won't even move like lips. Thus if the lips move, then that in itself will prove that they are neither rubies nor rose-leaves, but rather something beyond both. It should be noted that both hoñṭ hilnā and hoñṭ hilānā can refer to simple movement, and also to speaking. Bahadur Shah Zafar:

gużarte haiñ tujhe iz̤hār-e muddaʿā ke gumāñ
mirā jo hoñṭ bhī ai bad-gumān hiltā hai

[you suspect the expression of desire/intention
if even my lip, oh suspicious one, moves]

Thus in Mir's verse ṭuk hoñṭ hilā tū bhī has the implication both of conversation, and merely of moving the lips (for example, by smiling).

FWP:

SETS == EK
MOTIFS == SPEAKING
NAMES
TERMS

The ik bāt is enjoyably multivalent. It might mean 'one idea' out of the two hypotheses suggested in the first line (perhaps the beloved's lips might flutter like rose-leaves, thus proving themselves not to be emeralds). Or of course, as SRF notes, 'one idea' that rules out both hypotheses-- without suggesting any other. Or it might mean 'one utterance' from the beloved that would be uniquely authoritative, depending on what you said. And in all these cases, the little ik provides a full range of descriptive possibilities: the bāt in question could be 'single, particular, unique, excellent'.

As SRF notes, the idea that the beloved's movement of her lips would settle the question (about her lips) is particularly enjoyable. Here's another, even more piquant instance along those same lines:

{1373,3}.

Note for grammar fans: Nowadays ṭhahar would not in actual pronunciation rhyme with gużar and the other rhyme-words in this ghazal. For discussion, see {546,1}.