«فَكُّ رَقَبَةٍ»
Freeing a person from bondage.
Raqabah literally denotes a neck, but here freeing a neck means emancipating the enslaved person, not detaching a part of the body. A part stands for the person as a whole.
That is the juzʾiyyah (part-for-whole) relationship, the reverse of the preceding example. A complete answer states the mentioned part, the intended whole, and their relationship.