ScalaTest's normal approach for running suites of tests in parallel is to run different suites in parallel, but the tests of any one suite sequentially. This approach should provide sufficient distribution of the work load in most cases, but some suites may encapsulate multiple long-running tests. Such suites may dominate the execution time of the run. If so, mixing in this trait into just those suites will allow their long-running tests to run in parallel with each other, thereby helping to reduce the total time required to run an entire run.

Because this trait extends OneInstancePerTest, each test will be run its own instance of the suite's class. This trait overrides the runTests method. If no Distributor is passed to runTests, this trait's implementation simply invokes its supertrait OneInstancePerTest's implementation of runTests, which will run each test in its own instance sequentially. If a Distributor is passed, however, this traits' implementation of runTests will, for each test, wrap a new instance of the suite in a special wrapper suite that will invoke just that one test, and passes the wrapper suites to the Distributor. The thread or entity that takes a wrapper suite from the Distributor will invoke run on the wrapper suite, which will run just one test. In this way, different tests of a suite that mixes in ParallelTestExecution will run in parallel.