On certain gas turbine engines they have an inducer which is normally a rotary compressor. This feeds the axial vane compressor, which compresses the air and pumps it into the combustor can(s). Under ideal conditions the inducer will provide the necessary airflow into the axial vane compressor to maintain stable combustion. Under certain conditions the inducer can’t provide the necessary airflow to the axial vane compressor resulting in combustion instability. This causes the axial vane compressor to slow down at which time the delivery and requirements are again matched causing the combustion to come on stronger than usual. This causes the free turbine to come up to speed so fast as to transmit a very high torque into the transmission. As I previously indicated this load can come on so strong as to damage the driveline and the dynamic system of the helicopter.
I do not dispute the explanations above but it seems that there are differing opinions relative to compressor stall and they are all most likely correct. One point I would like to make, a long time ago I lived next to the North runway at LAX and when the 747s first went into service they would make loud explosive noises as they built up speed. I was told that this noise was caused by combustion instability and to solve this problem they incorporated blow in doors on the inlet of the engine, which solved the problems. The explanation was that there was a mismatch in the needs of the engine and the capability of the compressor to deliver the proper airflow and pressure to the combustor cans that is, until the 747 built up speed during take off.