The Apple //e originally did not support Double HiRes graphics (DHR). The engineers realized they could double their hires and lores X resolution shortly after the introduction of the machine, so the feature is only available on Revision B and later motherboards. However, to use DGR/DHR you needed the Extended 80-Column Card which added 64KB of RAM to the machine in the AUX slot and you had to close a jumper on the card to enable the feature.
While I've seen some screenshots of what happens when DHR is attempted on Rev A machines (basically a horizontally mangled normal HiRes mode), I became curious as to why it required a jumper since it appears to just fail visually otherwise.
Is there something in the firmware which is reflected by the jumper that would otherwise "protect" DHR on a non-Rev A machine? I'm just really curious what the jumper really does since the failure mode on a Rev A machine (with correct jumper setting) simply seems to be the horizontally smushed display.