What you are watching. Squares are power stations (generators), circles are demand buses (cities/industry), and the links are transmission lines. The simulator solves the linearised DC power-flow equations to find how much power runs through every line: where θ are bus voltage angles and B is the network susceptance matrix. When a line exceeds its thermal limit it trips — and its power instantly reroutes onto neighbouring lines, which may now overload too. That chain reaction is a cascading blackout, the mechanism behind the 2003 North-East America and 2006 European blackouts. Engineers design for the N−1 criterion: the grid must survive losing any single line. Push the demand high enough and even N−1 isn't enough.