From the CAA Training Standards Communication 1/2006.
Oscillatory Stalling
In May 2005 an instructor and student were on a training flight when the aircraft was seen to enter a spin and continue spinning until it impacted the ground, killing both occupants. The AAIB investigation stated "Although an unidentified control problem, loose artice or other distraction could not be eliminated as a contributing factor, it is considered that the likely scenario was that the aircraft entered an unintentional spin during an evercise involving "oscillatory stalling".
The term "oscillatory stalling" is not commonly used. No reference could be found in UK flight training publications, by either the AAIB investigators or CAA staff flight examiners. However, from other sources it is understood to describe the practise of maintaining the aircraft in a stalled condition whilst controlling wing drop with rudder.
Two points should be emphasised here: firstly the purpose of the stalling exercises on the PPL syllabus is to ensure that students can recognise the warning signs of the approaching (incipient) stall and the symptoms of the developed stall, and be able to recover safely from incipient and developed stalls. It is not necessary or adviseable, having demonstrated the signs and symptoms, for instructors to hold an aircraft in a developed stall, and students should never be taught to do so for the purposes of this exercise. Secondly, instructors are reminded that the rudder must only be used to maintain balance (prevent yaw) throughout the stalling exercise. The essential components of a spin are wings stalled with yaw and/or roll present. Deliberately inducing yaw with rudder to "pick up" a dropped wing, with the aircraft stalled, is tantamount to setting up for a spin.