Exactly what the man said, although I don't think "Shears" is the correct term. "Yields". perhaps?

"This link holds the airplane back- steam pressure comes up and at a certain strain this link shears at the weak spot and the aircraft moves down the track. Have to use one for every shot and every plane and load requires a different one."

Merrry Christmas, All
Les

Yes, shear stress is parallel to the cross section. Normal stress is perpendicular to the cross section, which is what this tension bar experiences.