Just a thought .... If the elevator has jammed then the trim tab will pitch the aircraft in the opposite sense (and not very powerfully). If the elevator's free to move then the trim will work in the correct sense as it is now effectively a servo tab. In some aircraft the trim switch alters the incidence of the whole tail (the Canberra is one such) and so is actually more powerfull than the elevator - hence the number of Canberras that speared in when they had a trim runaway.