The server can run in one of two modes, depending upon the load. It changes modes to accommodate the load most efficiently.
In high concurrency mode, after finishing the request, session threads give the connection to the keep-alive subsystem. In high concurrency mode, the keep-alive subsystem polls for new requests for all keep-alive connections.
When the server is started, it starts in low latency mode. When the load increases, the server moves to high concurrency mode. The decision to move from low latency mode to high concurrency mode and back again is made by the server, based on connection queue length, average total sessions, average idle sessions, and currently active and idle sessions.