Its an electromagnet that they have cooled down to 4 Kelvin with liquid helium. They take time to ‘wind up’ aka ‘ramping up to the desired/max field strength capable of the magnet’. They do this slowly because the magnet itself can crack if done too quickly, and many components are still affected by the strong magnetic field due to Lorentz forces. Also many components may be classified as ‘non-magnetic’ but still have some small amount of magnetism and can move when subjected to the extremely high magnetic fields. So, if the magnet is ‘quenched’ (all the helium shot out through a tube in the roof) then that process occurs in reverse, VERY quickly, potentially destroying many things. So its not like ‘cutting the power’ because the power is stored around the magnet itself by supercooled components creating a superconducting situation. Nonetheless, in case of harm coming to a person, techs should absolutely hit the quench switch. Not sure what happened to allow this guy in that room though