Magnetism is used on objects that have a U-shaped marking on them, signifying their magnetism. A U shape is drawn to pull objects together, which is often used for such purposes as moving an object onto a switch or moving an object in Chibiterasu's path to a different place. To move objects apart, an upside down U is drawn, a skill most often used to open heavy doors by pushing them apart. The skill is also used in battling a Black Imp, as it pulls the Imp's floating parts back together to make it attackable.
Magnetism is used by drawing a U shape. A normal U is used to draw objects together, while an upside down U is used to push them apart.
While traversing the Underground Ruins, Chibiterasu and Kurow come across a large statue of Kyokugami in one room. After restoring the god's constellation, his children appear to deliver Magnetism to Chibiterasu. Magnetism is quickly used in both its forms: two switches are pulled together to open a door, and the next door encountered is pushed apart with the technique's upside down version.
Spoilers end here.
"This new brush skill comes courtesy of the Children of Kyokugami. When you find a U drawn on objects, drawing a U will pull them together. You can also draw an upside-down U to push them apart." *Magnetism Tips* "The magnetic U mark can face any numbers of directions, so keep an eye out for them!"