I have no idea what the GP meant, but here's my take:
Set a minimal threshold of cyclomatic complexity. Below this threshold, function are assumed "so simple there are obviously no defects". Above this threshold you need tests, roughly prioritized in order of descending complexity.
It's not a bad idea, but that assumption of simplicity is a doozy. Also, figuring out cyclomatic complexity in Ruby (especially Rails apps) is ... hard. Especially if you want the computer to do it for you.
For instance, you can have a test result of 100% line coverage but if you have composed conditionals and you only check the first part of them... you are NOT checking all the paths of your code.
There are a couple of well-established advanced criteria like decision coverage, condition coverage, MC/DC to choose from if statement coverage isn't enough.