I decided to read up on it since it looks a bit mental but once you know why it makes a lot of sense.

Plasma is charged and needs to be contained with magnets because it’ll melt anything it touches.

To use the magnets uniformly on a donut shaped cloud of particles will cause the particles drift and hit the wall because the donut has particles traveling short distences close to the donut hole and long on the rim. The particle will eventually drift into the wall and destroy the reactor.

So fuck it, squash the donut and twist it around and now all of parts of a cross section traveling around the donut are traveling equally long distance.