"what you are seeing is the infamous "tennis racket" effect,"
Agreed,
This is a response which is normally observed on the 412 in a low density altitude enviroment and is related to the "tennis racket effect".
The amount of resistance occuring at the last bit of collective full down travel during single hydraulic operations will vary from one ship to the next dependant on the sum of the 4 main rotor blade's variance in built in blade-twist. The effect is also a product of the loads created by centrifugal force so, as rotor rpm reduces below 100% so does the resistance.
Hope this helps.