The spot welder is a resistance welding device that uses a large current through a pair of metal arms to heat two sheets of metal until they are permanently bonded via a weld. When the foot pedal is depressed, the arms squeeze together against the part and apply a current. The part will then be heated at that spot, and the parts will be welded together. This operation may be accompanied by a small spray of sparks. While this means that the spot weld is not optimally calibrated, this is perfectly normal, and your part will be fine.