This is a feature of ArcGIS Geoprocessing Framework, where the processing happens in a background (Non UI) Thread.
Starting with ArcGIS 10.0, ESRI introduced a new functionality in the GeoProcessing framework. This was an Opt-out functionality to run GeoProcessing tasks in a Background thread. This enabled the UI to remain responsive, while the tool ran in the background.
At first this was only a 32 bit process, but ArcGIS 10.1 introduced a 64 bit Background processing framework.