The most commonly used form of magnetic brain stimulation in clinical practice is transcranial magnetic stimulation (known as TMS). This makes use of Faraday's law of induction (if you remember from physics) to create current flow within the brain, which, in turn, excites the neurons (mostly cortical interneurons) over where the magnetic stimulation is applied.
Magnetic stimulation by itself is not considered a treatment option for brain tumours. However, as an example, we have used transcranial magnetic stimulation prior to brain tumour surgery. By stimulating the brain region by region, it is possible to map out the critical brain structures surrounding the tumour. In this way, these brain regions can be protected during the surgery.