A definite portion of brain tissue, ~1cm3 in volume, is teased by means of a directed, very strong and short-term (1/1000 second) magnetic impulse. If the brain tissue is exposed to a series of such TMS impulses (rTMS, r=repetitive), the activity in the irritated area ceases, for a few minutes, and various tests can be used to try to clarify the normal function(s) of the disconnected area. The practical usefulness of the method and its risks are hotly debated.
Cf. with tDCS.