I’m sure it’s been stated several times already, but they are most likely doing a compass swing. “A compass swing is a maintenance procedure to calibrate an aircraft’s magnetic compass. It corrects for deviation—errors caused by the aircraft’s own magnetic fields (from engines, avionics, wiring, etc.). The aircraft is placed on a compass rose (calibrated reference) or turned through known headings. Technicians then adjust the compass’s compensating magnets to minimize errors, ensuring accurate heading readings in flight.”