Blended testing strategy in agile. No test cases based on specification?

In a book I have, it says that for agile project, blended test strategy is the best, where it should use:

• Risk based strategy


• Reactive


• Automated regression

So I was about to implement in our project but now I realized - how can our testers verify the requirements? None of these strategies really uses functional specifition, unlike classical requirements-based one, which authors mention to have issues with agile approach because reuirements change. That makes sense, however, it means testers cannot really verify the functionality. Do I get that right or is there something I cannot see? So far on all projects I have been to, testers in the beginning of the sprint wrote test cases in advance, which does not go with these strategies.