The system in brief
The whole aim is superiority — numerical (a free man), qualitative (your best against their weakest), or positional (players staggered between the lines). The famous rules operationalize it: the marked pitch and half-space corridors, maximums like three players on a horizontal line and two on a vertical one, triangles and diamonds for support, the fifteen-pass idea to establish structure, and rondos and positional games as the training vehicles. And the line that says it all — move the opponent, not the ball.
What I keep and what I leave
I leave the rigid grid; a group of thirteen-year-olds doesn't need maximums-per-line to become good, and there's no single right way to play. But 'move the opponent, not the ball' is gold at every level — it turns passing from aimless keep-ball into a tool for creating space, which is a decision players can learn. And the rondo and positional-game formats are just excellent decision-rich practice, superiority baked in. So I steal the principle and the practice designs, and I skip the doctrine. That's how I use any big system: take the transferable idea, leave the parts built for a different team than mine.