It retards the timing, over fuels andalso jacks open the idle control valve to allow air into the cylinders when the throttle is closed. This results in fuel in the exhaust manifold which ignites and spins the turbo.
Depending on the system you can have positive boost at idle. However that would be a very aggressive system, mine at idle runs at barely any vacuum when running. It IS noticeable when running, contrary to what most nay-sayers erm... say.