When you run ad hoc jobs, with the exception of profiles, the Data Integration Service uses round robin to dispatch the first request directly to a worker service process.
Ad hoc jobs include mappings run from the Developer tool or previews, scorecards, or drill downs on profile results run from the Developer tool or Analyst tool.
To ensure faster throughput, the Data Integration Service bypasses the master service process. The worker service process creates a DTM instance within a separate DTM process to run the job. When you run additional ad hoc jobs from the same login, the Data Integration Service dispatches the requests to the same worker service process.