At the moment, NCrunch just considers NUnit explicit tests as ignored. The reasons for this are largely historical (the first versions of NCrunch did not support manual or part continuous), although they don't really apply any more. Ideally the behaviour should be that Explicit tests are not run automatically with NCrunch, and are run manually instead. There is a task in the backlog to address this in future.
If possible, I'd recommend using an alternative to the Explicit attribute for the time being. If you're prepared to fully rely on NCrunch to run your tests, you can create your own custom category that specifies tests which should not be run continuously (using a custom engine mode).
Cheers,
Remco