الَّذِي أَنْقَضَ ظَهْرَكَ
Which had weighed heavily upon your back. (الشرح: 3)
Alladhī modifies the previously mentioned wizraka. Its relative clause, anqaḍa ẓahraka, describes the severity of that burden more vividly than the isolated noun would. It is therefore an explanation of an existing referent, not a new independent divine favor.
The explanatory effect resembles clarification after initial generality, though grammatical classification should remain distinct from rhetorical function. A relative clause has a specific syntactic role; the additional words are rhetorically useful insofar as they make the earlier burden more intelligible and affecting.