Aser,
little bit rusty on 212/412 AFCUs now, but as I recall, the topping adjustment is accomplished by moving a cam on the AFCU assembly, which then determines how far a mechanical arm attached to the throttle can move. To carry out topping checks without the need to get airborne, and to conserve engine life, it is possible to insert a bush of sorts (the part power stop) to this cam assembly, that limits the movement of the arm even further. The theory is that if you can achieve 92-92.8% g with this 'obstruction' in place, with it removed you should be able to achieve the normal topping figures (102.4 etc depending on the engine series).
In effect, this achieves much the same result as the means by which OEI training is carried out; a throttle is backed off to reduce the topping value and then frictioned tightly to prevent inadvertent movement while the other throttle is then used for simulating failures.