I will try to explain what I am saying using the commented video. It is rather old as now the movements we use are very small to avoid losing the coverage of the central line, but maybe for that reason the explanations will be easier to follow. I have chosen only the first action that goes from 00 to 08 seconds.
All that I pretend here is to explain how the theory works and it is used to explain actual fencing. I have eliminated most of the words, and expressions we use in Spanish to make it easier to understand and not too long to read. Translating it into English it is not easy and some words may sound strange in English. I apologize if I am not able to find better words or sentences, I am not a native English speaker and often this is necessary to make good translations of concepts.
In the pictures included I am only using the circle on the ground, the same circle may be used at waist level and shoulder level, forming different lines and angles. The ability to read the lines and angles is related with practical knowledge. The better a fencer is, the better he understands the theory and knows how to use it. Without practical experience the theory is worthless.
Seconds 00 to 08
As we are starting to fence we are too far from each other so we have to use the sword in an obtuse angle. Diego tries a couple of times to close distance extending the arm occupying the Right Angle but he doesn’t find the proper mean – distance and angle – and keeps on moving towards the left side (or sword side in Destreza).
Line AB is the common diameter and line CB is the particular diameter Diego is looking for. The problem is that I am out of the circle and the Mean of Proportion, using an obtuse angle stance which makes impossible for him to control my sword. That is why in the following moments he thrusts in the Right Angle.
During second 005 Diego gets the Mean of Proportion after I lower somewhat the sword to give him the chance to extend the arm and getting for me the chance to get some control of the sword. He does it and delivers the thrust but he is careful so even though I deviate the sword – desvio – he immediately retreats the sword and steps left with an horizontal step and losing distance I can’t get the bind in the inside – atajo interior -.
The picture shows the moment I make the desvio. I would have bound if I could but he is careful and doesn’t extend completely the body so I cannot finish the bind. If I would have done it would have been very dangerous for me as he could have disengage under my sword and thrust at my chest.
In the picture the red line AB shows the common diameter, the black one BC my particular diameter X is the Proportional Mean in which I am and Z is the Proportionate Mean from were I could have thrust if I would have got there.
Many readers would ask why the common diameter is on the left and not between us. The answer is simple: because of the shoulders alignment: what we call the aspects. By turning the shoulders the circle moves without moving the feet. If Diego shoulders had been completely profiled I would have thrust him with a cuarto de circlulo under his arm, but he is careful not to do that preventing it.
From another point of view looking at the blade we can see how I do not have enough degrees on his sword to bind, for the reasons explained. That is why I can only use the desvio but I can’t proceed to execute the bind.
I attach the complete video so you can see what I'm talking about
(to be continued)