A relative clause may function as a means of reinforcing an assertion (ذريعة إلى تحقيق الخبر). It recalls an established or vividly presented circumstance that makes the following report more forceful and intelligible.
The poem discussed on printed page 153 starts with «إن التي ضربت بيتاً مهاجرة ...». The clause evokes a woman's migration and establishment of a home before the report concerning the alteration of affection. This prepares the hearer to appreciate the report's significance.
Taḥqīq (تحقيق) should not automatically be interpreted as a formal logical demonstration. Here it refers to a rhetorical strengthening of the assertion within the discourse.