This is similar to the classic example in Halliday and Resnick (or at least, it was in the Great Eggplant version), where there are two sections of rope, identical material, one tied to the ceiling, then to a mass, and the other hanging from the mass. [![enter image description here][1]][1]

If you pull slowly and firmly, the combination of your pull and the gravitational force $Mass*g$ will cause the upper rope to break first. If you pull with a sudden strong jerk, the lower rope will break because it can't withstand the peak force.

My money's on the same thing in your case: the rod shatters before it can transmit the force of the blow to the glasses. [1]: https://i.sstatic.net/rNrXd.jpg

This is similar to the classic example in Halliday and Resnick (or at least, it was in the Great Eggplant version), where there are two sections of rope, identical material, one tied to the ceiling, then to a mass, and the other hanging from the mass.

If you pull slowly and firmly, the combination of your pull and the gravitational force $Mass*g$ will cause the upper rope to break first. If you pull with a sudden strong jerk, the lower rope will break because it can't withstand the peak force.

EDIT: here's a photo of an experimental setup to accompany that H&R discussion: weights and ropes . As Owen B mentioned, inertia plays a key role here. Backtrack from that link to find the section of H&R of interest.

My money's on the same thing in your case: the rod shatters before it can transmit the force of the blow to the glasses.