Usually a single edged cut and thrust weapon, such as a sabre, has more than one edge. The top third of the back of the blade is often given an edge to add to the blades thrusting ability. Such an edge is known as the false edge.
The false edge in tango is blindly practising technique, ingraining your movements with the soulless movements of an automaton. Yes your movements will have gained an edge in their precision but your dancing will lose something in the way of feeling. If you want to find feeling when you dance then it is essential that you have feeling when you practise.