(at the second visit on 07/03/2014)
A new improved prototype is born:
DETACHING: the person in the wheelchair gives to order to stand still and puts the hook in the loop by steering his wheelchair towards it. Then he orders the dog to detach himself by shouting "collier!". The collar stays on the hook now. The last order is the one to collapse the arm.
ATTACHING: the dog gets the order to pull out the arm attached to the wheelchair. The the order "collier!" is given. The dog put his head through the collar. Now the person in the wheelchair can detach the loop from the hook if the dog is standing still.
This system could perhaps work with a magnet as well.