Surge is NOT as described but when the COMPRESSOR side cannot produce ENOUGH airflow, due to bad ratio between TURBINE and COMPRESSOR sizes.
The surge effect is caused by the air speed stalling due to not having enough power transmitted through shaft to drive it sufficiently hard, due to amongst other things too large a compressor side unable to build boost, which is continually being used.
All turbo's have a window or engine load/rpm where they operate best, below this pointy you can get a surge situation whereby the flow is unstable due to the turbo not being able to supply enough air.
When the mass flow rate through a compressor is reduced while maintaining a constant pressure ratio a point is reached where local flow reversal occurs in the boundary layers, if the flow point is further reduced complete flow reversal occurs.
This will relieve the adverse pressure gradient until a new flow regime is established at a lower pressure ratio, the flow will then build up again to the initial condition and this flow instability will continue at a fixed frequency and can be quite violent and is called surge.
It is more prone on turbo's that have a small operating window.
Put simply there are various reason for surge but most common is bad match of comp/turbine housing sizes whereby the shaft connecting the two cannot transmit enough force through to the compressor side due to compressor being to large, and power transmit not being great enough from turbine impeller.
tabetha