إِذْ يُوحِى رَبُّكَ إِلَى ٱلْمَلَٰٓئِكَةِ أَنِّى مَعَكُمْ فَثَبِّتُوا۟ ٱلَّذِينَ ءَامَنُوا۟ ۚ سَأُلْقِى فِى قُلُوبِ ٱلَّذِينَ كَفَرُوا۟ ٱلرُّعْبَ فَٱضْرِبُوا۟ فَوْقَ ٱلْأَعْنَاقِ وَٱضْرِبُوا۟ مِنْهُمْ كُلَّ بَنَانٍ
Important Words:
فوق (upper part) is derived from فاق. They say فاقھم i.e. he was or became above them. So فوق means, the location or place that is above or over a person or a thing. They say فوقه رأسه i.e. his upper part is his head (Lane).
Commentary:
This verse shows that the angels are, as it were, intermediaries between God and men. It is through them that God’sassistance comes to men.
The verse also hints that there are some angels whose function is to strengthen the hearts of good men and make them firm, while there are others who are deputed to cast terror into the hearts of wicked men. So God bade the former class of angels to strengthen the hearts of the believers and told them that He would cast terror into the hearts of the disbelievers through the other class of angels.
God instructed the believers to strike the disbelievers at "the upper parts of their necks" because the upper part of the neck, i.e. that which is just below the head, is considered to be the best point for dealing an effective blow with the sword. This order related to such of the enemy as deserved to be killed and without killing whom the Muslims could not defend themselves. The striking on the “finger-tips” was meant to incapacitate the enemy and make them unfit for fighting. This order thus related to those of the enemy who, so far as possible, were to be spared with the infliction of minimum injury. Both these orders were faithfully carried out, for although the Muslims were very weak at Badr, they were able to kill seventy of the disbelievers, mostly leaders, and take as many prisoners, while the enemy could kill only fourteen of them.
The upper part of the neck which is just below the head and is considered to be most vulnerable for dealing an effective blow with the sword.