Answer: After a minute or two, when feeling stuck, to work on easier problems before going back to the hard one.
Step-by-step explanation:
The hard-start technique means, first, looking over the entire test to be able to identify the most difficult problems.
Then, start working on one of the hard problems, only for a minute or two, meaning, until we start feeling stuck. That a stopping moment, to work on easier problems which help re-boost the confidence required to later go back to the hard problem.