The evidence is that the system works very well but there is some discussion about exactly how.
One concept is that the pulsed magnetic field impacts any magnetically sensitive elements in the cancer cells, causing them to oscillate. Cancer cells have more transferrin (iron) receptors in the cell wall so that they take up more iron than non-cancerous cells. This movement of particles in the cell generates heat as the particles push their way back and forth through the cell fluids. The heat generated is unable to escape from the cell so that it accumulates, little by little. When the temperature of the cell reaches 43 Centigrade the cell dies.
Another concept is that the pulsed magnetic field disrupts the Ferro-Sulphur clusters on the mitochondria of the cancer cell leading to cell death.
However, this discussion while interesting does not change the fact that cancer cells die. For more than one hundred years we did not know how aspirin relieves pain. Government authorities, such as the US FDA, only ask if the treatment is safe and effective. They do not ask how it works.