There are times when it would be better for another engine to run part of a query, e.g. if there's a resource light, but slow/long blocking section of a job that would be better run outside of Thor, so as not to occupy a Thor queue slot.
The example that led to this JIRA was a section of a job that was spending some hours making SOAP calls. Other than blocking on the remote service, the Thor was practically idle.
I'm not sure what the syntax would look like, 1st thought is overloading INDEPENDENT, e.g. :