I think the issue they were getting at is code like this:

    function doSomething(doSomethingElse) {
      internalState.setUp();
      doSomethingElse();
      internalState.tearDown();
    }

Right now, you can safely assume this whole thing will execute to completion without anything else happening in the interim, so a lot of existing JS makes that assumption. If `doSomethingElse` can yield while other code executes the assumption becomes invalid.