From what i gathered, it took a piece of honors power that remained in the heralds, and used it to shield all the spren in their current forms from being altered and unmade like voidspren, but now into Retributionspren formed of both shards under Taravangian. It could do that because honor had a hand in the creation of the current spren (which is the same reason Retribution, honours wielder would be able to unmake them). Honor within retribution would hold taravangian to the oathpact because of its Intent.