In 32-bit ArcGIS for Desktop, the background geoprocessing function works by using a separate 64-bit process to perform the analysis, which is why 64-bit Background Geoprocessing must be installed separately. When invoked, it is seen in Task Manager as two RuntimeLocalServer.exe processes.
Since ArcGIS Pro is itself a 64-bit application, a separate process for background geoprocessing is no longer needed. Tools are run asynchronously, not on a separate task that appears in Task Manager, and allow work to continue in the application while they execute.
As of this writing, additional functionality to run tools in a separate process is being considered.
The in memory workspace option is available for models and scripts in which it is used as intermediate storage for tools chained in sequence. When running tools individually in the Geoprocessing pane or from a Python window, the project default geodatabase is substituted for the in_memory workspace.