The Robinson safety notice says it all - the pilot was unaware of the increase in power required as the IAS fell and ETL was lost - subconsciously probably raising the lever to maintain height and slowly decaying the Nr (thus further reducing the TR effectiveness).
Then, when it starts to spin, a reluctance to lower the lever sets off the Nr warner, the Nr droops further, increasing the RoD and the yaw because the TR isn't spinning fast enough. It takes him nearly 2000' to do what he should have done in the first place - lower the lever!