A Note On Allah's Shin

S 68:42  On a day it is uncovered from a leg and they are called to sujud but they cannot.

We are not actually told who or what uncovers the shin.

The Jalalayn on this verse:

Mention the day when the shank is bared an expression denoting the severity of the predicament during the reckoning and the requital on the Day of Resurrection one says kashafati’l-harbu ‘an sāqin ‘the war has bared its shank’ to mean that it has intensified and they are summoned to prostrate themselves as a test of their faith but they will not be able to do so — their backs will become as stiff as a brick wall.

The Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs:

(On the day when it befalleth in earnest) a matter to which they were blind in the life of the world; it is also said this means: when a severe, horrific matter is revealed; and it is also said that this means: when a sign between them and their Lord is revealed, (and they are ordered to prostrate themselves) this is because they said: By Allah, our Lord, we were not idolaters or hypocrites (but are not able) to prostrate, their bodies remaining erect like solid fortresses, altafsir.com

Ibn Kathir:

(The Day when the Shin shall be laid bare and they shall be called to prostrate themselves, but they shall not be able to do so.) meaning, the Day of Judgement and the horrors, earthquakes, trials, tests and great matters that will occur during it. Al-Bukhari recorded that Abu Sa`id Al-Khudri said that he heard the Prophet saying,

«يَكْشِفُ رَبُّنَا عَنْ سَاقِهِ، فَيَسْجُدُ لَهُ كُلُّ مُؤْمِنٍ وَمُؤْمِنَةٍ، وَيَبْقَى مَنْ كَانَ يَسْجُدُ فِي الدُّنْيَا رِيَاءً وَسُمْعَةً، فَيَذْهَبُ لِيَسْجُدَ، فَيَعُودُ ظَهْرُهُ طَبَقًا وَاحِدًا»

(Our Lord will reveal His Shin, and every believing male and female will prostrate to Him. The only people who will remain standing are those who prostrated in the worldly life only to be seen and heard (showing off). This type of person will try to prostrate at that time, but his back will made to be one stiff plate (the bone will not bend or flex).)'' This Hadith was recorded in the Two Sahihs and other books from different routes of transmission with various wordings. It is a long Hadith that is very popular. Concerning Allah's statement,

خَـشِعَةً أَبْصَـرُهُمْ تَرْهَقُهُمْ ذِلَّةٌ

(Their eyes will be cast down and ignominy will cover them;) means, in the final abode, due to their crimes and arrogance in the worldly life. Thus they will be punished with the opposite of what they did. When they were called to prostrate in the worldly life, they refused to do so even though they were healthy and secure. Therefore, they will be punished with the lack of ability to do so in the Hereafter. When the Almighty Lord makes Himself visible (before the believers), then the believers will fall down in prostration to Him, but no one of the disbelievers and hypocrites will be able to prostrate. rather, their backs will become one plate. Everytime one of them attempts to prostrate, he will bow his neck but will not be able to prostrate. Ibn Al Kathir commentary for verses 68.42-47

"Then the Almighty will come to them in a shape other than the one which they saw the first time, and He will say, 'I am your Lord,' and they will say, 'You are not our Lord.' And none will speak: to Him then but the Prophets, and then it will be said to them, 'Do you know any sign by which you can recognize Him?' They will say. 'The Shin,' and so Allah will then uncover His Shin whereupon every believer will prostrate before Him and there will remain those who used to prostrate before Him just for showing off and for gaining good reputation."

https://sunnah.com/bukhari/97/65 

So, according to the hadith, Allah's shin is a means of identifying it. Muslims presumably will have seen it before.

Now, as saq can be used for the whole leg, why might Allah be showing its 'awrah?

'Awrah/Nudity