That only works if the code was written in a semi-digestible format; I've met my fair share of monstrosities that were propped up by race conditions -- you're not going to briskly digest them in a week (especially if you're in the unlucky circumstance of only having a single breakpoint per run and the engineers who had any understanding of it haven't worked there in 20 years).
Test cases can be a good place to start because they are usually clustered around business critical functionality. Or problematic legacy code that needs to work :)
I wish that was an option for at least half the codebases I've worked on. Tangent: you'd be surprised how many life threatening codebases have little to no tests at all.
Test cases? I still feel lucky to discover that a new (to me) project is even using source code control, let alone has some kind of test suite, let alone one that it currently passes.
