It was an expression to take one’s arm in that position so that it can reach one’s armpit. He was first ordered to insert His hand inside His collar and then stretch it to his armpit the way its done in that posture of fear where one crosses his arms to his opposite armpits.
فَلَمَّا أَتَاهَا نُودِی مِنْ شَاطِئِ الْوَادِ الْأَيْمَنِ فِی الْبُقْعَةِ الْمُبَارَكَةِ مِنَ الشَّجَرَةِ أَنْ يَا مُوسَى إِنِّیْ أَنَا اللَّهُ رَبُّ الْعَالَمِيْنَ. وَأَنْ أَلْقِ عَصَاكَ فَلَمَّا رَآهَا تَهْتَزُّ كَأَنَّهَا جَآنٌّ وَّلَّى مُدْبِرًا وَّلَمْ يُعَقِّبْ يَا مُوسَى أَقْبِلْ وَلَا تَخَفْ إِنَّكَ مِنَ الْآمِنِيْنَ. اُسْلُكْ يَدَكَ فِیْ جَيْبِكَ تَخْرُجْ بَيْضَآءَ مِنْ غَيْرِ سُوْءٍ وَّاضْمُمْ إِلَيْكَ جَنَاحَكَ مِنَ الرَّهْبِ فَذَانِكَ بُرْهَانَانِ مِنْ رَّبِّكَ إِلَى فِرْعَوْنَ وَمَلَئِهِ إِنَّهُمْ كَانُوْا قَوْمًا فَاسِقِيْنَ.
Then when Moses reached there, he heard a voice from a tree in the valley of Ayman found in that blessed land: “Moses! I am God; Lord of the worlds and this also that place your staff [on the ground].” Then when Moses saw it slithering like a serpent, he turned and fled and did not even look back. God said: “Moses! Come forward and fear not. You are absolutely secure. Insert your hand into the opening of your garment [at the chest;] it will come out [shining] white without any ailment and for this draw your arm towards yourself as is done in fright. So, these are two signs from your Lord to go to the Pharaoh and his courtiers. In reality, they are a very disobedient people.(28:30-32)
(Excerpt from Quranic Exegesis: Javed Ahmed Ghamidi)
(Translated by Dr. Shehzad Saleem)