The Arabic language, the word رَقَبَةٍ (a neck) refers to the base of the neck, not the neck itself. So, this word is used because a person’s life is possessed from his neck and often is constrained by it.

فَكُّ رَقَبَةٍ (rescue someone from slavery): The devoted people can buy slaves to free them. This is the first manifestation of releasing slaves in history. Islam came to end slavery of Man to his brother Man gradually. Hence, Islam made the freeing a slave as an expiation for some sins, additionally, in Islam, one of the alms giving (Zakat) outlet is to free a slave. Allah (swt) in this verse describes freeing slaves as breaking through difficulties pass as a meaning of breaking through sins, which are as mountains, to be vanished and dissolved. Therefore, whoever sins, then, seeks Allah’s forgiveness and gives a charity, consequently, he is breaking through the sins and smashing them.