Can't fully get my head around that but it kinda seems to me that it isolates the midfielders and the defenders aren't involved in the triangular passing?
What the author is basically saying is that:
1. on the left side of the pitch there is one triangle consisting of the left fullback, the left midfielder and the left forward.
2. on the right side of the pitch there is one triangle consisting of the right fullback, the right midfielder and the right forward.
That way Barcelona don't often get outnumbered in midfield just beacause they have only 3 players there playing the ball between them. They also have 1 forward and 1 fullback participating in the passing game - in other words always 5 players quite close to each other.
If the opposition closes them out they simply shift quickly from side to side through a cross ball, and there they have already formed their second triangle, and can continue penetrating the opposition's defence.
Don't know if that clears anything out for you, but it did for me!
