فَكُّ رَقَبَةٍ
In the widespread text of the muṣḥaf (the transmission of Ḥafṣ from ʿĀṣim) the word is in the مصدر (verbal noun) form: fakku raqabatin — "the loosing of a neck", and in the next verse iṭʿāmun — "feeding". On this reading verses 13-16 are the answer given to the question in verse 12: what is the ascent? — It is the loosing of a neck, or feeding.
From some of the other imāms of the readings a verbal reading is transmitted: fakka raqabatan — "he loosed a neck", aw aṭʿama — "or he fed". On this reading the verses attach to the negation in verse 11: he did not throw himself at the ascent — he neither loosed a neck nor fed anyone.
I give no definite list as to which imām adopted which reading; the detail is recorded in the sources, and I do not convey an ascription I am not sure of.
The difference in sense is real but small: on the verbal-noun reading the ascent is defined, on the verbal reading the failure to climb it is exemplified. On both, the content of the ascent is the same. Besides, the verbal reading also solves the problem of "lā + perfect" discussed above at verse 11: the negation turns out to have been repeated.
The concrete uses of the word: to untie a knot, a joint's coming out of place (infikāk), to open a lock, to separate two things from one another. The English verbs "to undo" and "to unfasten" cover this field well.
The choice of word is meaningful. The verse does not use the expression the Qurʾān uses elsewhere for "to free a slave". In contexts of expiation the Qurʾān says تحرير رقبة (taḥrīru raqabatin — to make a neck free) (al-Nisāʾ 4/92, al-Māʾida 5/89, al-Mujādala 58/3). Taḥrīr comes from the root of ḥurr (free) and conveys a legal act.
Here, though, we have fakk — to loose a bond. Not a legal but a physical image: the opening of the tie of something that has been tied.
This difference agrees with the sūra's being Meccan. In Mecca there was no legal regulation about slavery, no institution of expiation. The verse is not describing a legal act; it is describing an act of loosing a bond.
Root: ر-ق-ب. The original sense of the root is to watch, to observe, to wait. Raqīb — one who watches; one of the names of God. Murāqaba — surveillance. Taraqqub — to await.
The connection is this: the neck is the axis on which the head turns and towards which the gaze is directed. One who watches stretches his neck, turns it. The lexicographers establish the bond in this way.
The use of the neck for a slave. In Arabic it is common for a part of a person to be mentioned instead of the whole, and the part chosen for a slave is the neck. The reason is concrete: a bond, a ring or a rope used to be put on the necks of captives and slaves. The neck is the place where bondage is seen.
This completes the image of the verse: fakku raqaba — the loosing of the bond on a neck. Read together, the words yield not an abstract legal act but a scene that can be touched: the opening of the ring on someone's neck.
The Qurʾān uses this word too in one of the places to which alms are to be spent: "…and for the necks (fi'l-riqāb)" (al-Tawba 9/60). There too the same image is present.