How does Father prevent the use of alchemy in Fullmetal Alchemist?

Question

I have recently started rewatching FMA brotherhood, and noticed some things I was confused about concerning Father.

He uses the pipes to spread the Philosopher's Stone beneath the nation of Amestris. The purpose behind this is to act as a barrier between the tectonic energy of the earth's crust, where Amestrian Alchemy gets its energy from, so that he can stop others from using Alchemy at any time.

Emphasis mine.

And also

he placed himself under what would become Central Command and used himself as a buffer against tectonic energy so alchemists would actually use energy derived from the souls of Xerxes contained within himself.

How is Father able to stop people from performing alchemy? And if the people in Amestris were partly using a philosopher's stone, wouldn't they be able to bypass the laws of equivalent exchange?

Answer 1

Scar draws a picture in one episode. Imagine philosophers stones Father had seeded all over the place as sort of inactive circle (Or open door.) You can walk into the circle, you can draw on that circle, you can dig into the circle. It is not active. You can use the energy from below the circle. Less than you normally would, but you can do it. Only after Father activates the circle, the flow stops.

Imagine open submarine door. Fishes swim in, fishes swim out (fishes are alchemists here). They have to pass through the doorway into the interior and there is not as much water inside the submarine as in the sea, but fishes can still swim and they can continue to lead their lives. Those hatched in the submarine which never left it don't even know it can be done the other way. People from Xing noticed that something is wrong (they are humans that swam into submarine from the sea) but they can't do anything about it.

One day, the door of the submarine closes and pumps start pumping the water out. In a few moments, the interior of the submarine is completely dry. What happened? Father activated his circle made from philosophers stones and cut access to alchemy to all alchemists. The fishes can't breathe! They can't live! But people of Xing and Hohenheim can, because they know the way to breathe air, not water. They have lungs, not gills.