if the person tries to stop it from happening it makes it worse, the movements and duration..
you have to let them do what they need to do, time it while its happening if it gets longer that 5-7 minutes, if they hit their head, or are not coherent, then you have to call 911.
they have to lay on their back and make sure they have nothing in their mouth or anything they can get hurt near them. do not move them or stop them from moving.