1. PRINCIPLE:
For given symmetric circuit, you can divide the circuit along its symmetry plane.
Then use symmetric excitations and symmetric load:
If you connect both part (the part you can see and the specular part removed from the circuit) the same voltage: in a symmetric circuit no current flows, for this reason it is equivalent to close your circuit with an open circuit at the cutting plane.
If you excite with the opposite voltage, you will have a short circuit since the current will be maximum.
If you have this circuit, you can divide here, obtainig the circuit of next slide

2. When Even this line is closed on a Open Circuit, i.e. Current=0
When Even this line is closed on a Open Circuit, i.e. Voltage=0
When Even this generator erogates V/2
When Even this generator erogates -V/2
“B” side is the side you can see
“A” side is the side removed (the specular)