Hi Aje,
A stall is when the airflow separates from the airfoil, this also applies to the blades of the engine compressor, which are basically airfoils. When the engine is at idle thrust there is the risk of having too much air pressure accumulated at the front stages of the compressor (the engine is not producing thrust so it is not "swallowing" so much air), something that can slowdown the total speed of the airflow that passes through the blades, causing a stall on them. Usually there is a dedicated valve that opens when the engine is at idle regime, to guarantee a minimum speed on the airflow through the blades by giving the air flow an easy way out (the valves usually are at the sides of the engine, not at the end of it, if I recall correctly). The engine anti ice valves also open this idle valve to guarantee the flow of air (this same vale or another dedicated one, depends on the engine), that is why the procedure.
I was told to do this same thing when I flew the MD80 if I was going to encounter turbulence, as a way of protecting the engine from a possible stall (JTD engines in the MD80 didn't come with FADECS).
Hope this clarifies.