How did Mei Chang use Alchemy when Father stopped it?

Question

When father stopped Alchemy , Ed and Al were unable to use it. But mei chang still used it.

How is this possible?

Answer 1

it's because Mei used Alkahestry

Alkahestry (錬丹術, Rentanjutsu), also called Purification Arts (連弾主, Rendan-nushi), refers to the slightly different form of Alchemy used in the country of Xing. Alkahestry differs from Alchemy in both its practice and its goal. Whereas Amestrian Alchemy claims to have its roots in the energy of tectonic shifts and practices manipulation of matter toward scientifically practical ends, Alkahestry is centered on a concept called the "Dragon's Pulse" which speaks of the Earth itself having a constant flow of chi (life energy) which flows metaphorically from the tops of mountains down to the land, nourishing everything it passes with that energy as does blood coursing through the veins.

Following the events in Xerxes, Van Hohenheim went East to Xing and helped develop Alkahestry. Father on the other hand headed west to Amestris and Developed Alchemy purposely to be flawed, with the difference between them (being discovered by Scar or maybe his brother) being that Father was the source of Alchemy.

After the circle is activated, Scar explains to Lan Fan how his brother had learned about alchemy and how Alkahestry had intrigued him. Scar's brother had been mystified as to why the National Library contained no books on the subject and was forced to rely on caravans from Xing to learn about Alkahestry. In his studies, he discovered to his horror that alchemy did not come from the energy of tectonic plate movement, but rather from a far more sinister source.

Source: Scar/History > March Toward the Future (5th paragraph)

The Wiki points to Father's wiki entry as the "far more sinister source"

However, to ensure his teachings would not be used against him, 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.

Source: Father > Part in the Story (6th Paragraph)

Also pointed out on the wiki. while Mei is the only one to use Alkahestry. Scar's Alchemy uses alkahestry principles allowing him to use it while Alchemy was sealed