not sure if it's semantics but the third pilot didn't know. he figured out the problem first then fixed the situation. then again, he wasn't busy flying the plane.
semp
The jumpseat pilot didn't "know"? He clearly
knew the long standing Boeing procedure for Runaway Stabilizer Trim. So yeah he "knew" the procedure.
What he did can be summed up succinctly as Analysing the Situation and Taking the Proper Action. That's a phrase most professional pilots have heard many times during their careers. The jumpseater analyzed the situation: Runaway Stab Trim. While he didn't actually throw the switches, he informed the PIC and SIC of the correct, proper action which he clearly KNEW. And which the PIC/SIC apparently DID NOT.
Just as an aside as well: the idea that the jumpseater
only figured out the problem because
"he wasn't busy flying the airplane".
Guess what! When you sew on that fourth stripe, you are expected to be able to simultaneously fly the aircraft, analyze the situation and take the proper action. It's part of the deal, see? That's what Captains DO. Captains are not supposed to have to rely on a knowledgeable jumpseat pilot to save the day.
Or as they say in the biz...."It's why you get paid the big bucks."