FWP:
SETS == HANUZ; WORDPLAY
On the idiomatic yak-qalam construction, see {11,1}. As the commentators note, this is a verse based on wordplay. In fact wordplay is all it's based on; there's literally nothing else in it except a fancy description of a 'hot' footprint in the desert.
One of the pleasures of working with commentators is their unpredictable outbursts. (I certainly have mine!) Nazm has an enjoyable outburst about this verse. He seems especially provoked by the phrase yak-qalam and the verse's ostentatious wordplay. He goes on at length, giving hard-to-translate examples that I have omitted, and declaiming about how vulgar and low-class it is to engage in fancy punning and competitive word-games. This, he says, is what the loafers and common people do, and not what the 'proper' ghazal should do. Nazm was a Lakhnavi; for a more detailed account of what he describes as Lucknow street culture, see Harcourt and Hussain; if you know Urdu well, you can go straight to the source, Sharar.
As usual, hanuuz (on this see {3,4}) has the two senses of 'still', emphasizing long duration, and 'now', emphasizing newness or immediacy. And what is it that the 'still' and 'now' help us to observe? The traces of the lover's passage through the desert; or, as Faruqi points out, the beloved's passage (though it's hard to imagine her as walking around in the desert). The 'heat of movement' might come from the lover's feverish passion, or the beloved's devastatingly 'hot' way of walking, or perhaps from a sheer power of movement that generates its own form of heat, like an engine running at high speed.
In any case, merely the act of lover's
or beloved's walking through the desert was too much for the desert to bear;
the desert now has charred, glowing embers in footprint patterns all over it. The inflammable
sandy desert, when exposed to the movement of human passions, caved in immediately and entirely,
and resembled something as frail and destroyed as a burnt, or literally (and
more graphically) 'fire-stricken', paper. Compare
the even more extreme (and effective!) {5,4}, in which even
a 'passing thought' of wildness/madness inadvertently burnt up the whole desert.
Nazm:
In this verse the author has used the term yak-qalam for the wordplay with 'paper'. In the poetry of that time, they consider wordplay too to be a verbal device, and they call it 'wordplay' when they use one word such that it would have some relationship and affinity, merely verbal, with another.... In short, there's no doubt that in such cases sometimes this wordplay, sometimes .zil((a , seems good, but they carried it to such lengths that in their search for .zila(( they have no thought for elegance of meaning [;husn-e ma((nii] or for simplicity of words.... Undoubtedly this is to be rejected; it is a verbal device invented [nikaalii hu))ii] by the common people [baazaarii]; literary people [ahl-e adab] have nowhere mentioned it. When the loafers [lau;N;De] of the city assemble, they compose .zila(( .... These very people have, in mushairahs and gatherings [majlis], incited the poets to such wordplay, and drawn them in this direction. (68-69)
== Nazm page 68; Nazm page 69; Nazm page 70