The idea is that once SQL Server has created enough threads to service the peak workload, it should not then trim worker threads (release them to the OS) after 15 minutes or so of them not being needed.
The idle worker threads will continue to use resources (e.g. memory) but there will not be the burst of
THREADPOOL waits when more workers are suddenly required. Apparently this can be of assistance when using Always On Availability Groups.
What is this undocumented trace flag, and how does it work?