It's the EGR valve working. I see this happening when I let go of the throttle on the vacuum gauge. When the EGR valve opens up, the engine lets exhaust gases in and reduces vacuum in the intake manifold. This allows the engine to turn freely and increases rpm even though no fuel is added. It's a way to save fuel and to meet emissions laws.
At least this is what I think. EGR only works when the engine is at operating temp.