In the last decades, customers and regulators are continuously imposing more stringent requirements on safety, emissions and fuel consumption, pushing the manufacturers to optimise every single phase of the engine operations. Many goals in design point operations and off-design ones have already been reached. However, in order to meet the requirements on ground start and altitude relight capabilities, the sub-idle regime performance still needs to be fully comprehended. In this regime, the engine is operating at a speed below the idle one and, since these are extreme off-design conditions, innovative approaches to create high-fidelity prediction tools are required. For this purpose, one of the main challenges is to further understand the compressor behaviour.