Hi Jesus,
TestLeft itself does not require active desktop session to execute. Also if your code in TestLeft drives unattended actions (i.e. those that does not have user interface), for example, creates and manipulates COM objects, than active desktop session is also not required.
But active desktop session is required if your test code drives regular desktop application with UI. This is required not because of TestLeft, but due to essential Windows OS requirement. Without active desktop session, GUI application does not render its UI and consequently, TestLeft fails to control it.
