[garethadams]

Think of it the other way around.

The top brick can be placed so that its centre of mass is positioned over the edge of the brick below. This two block system is stable.

The two block system can be placed on a third block so that the centre of mass of the two blocks is positioned over the edge of the third block. This three block system is then stable.

From there, it's turtles all the way down

[Dylan16807]

But the extension mechanism has nothing to do with placing the top brick. It seems like a mistake in the writeup.

[turtlepower]

Correct.