This is called piggy-backing and it has several uses. Early PCs would stack/piggy-back memory chips to increase capacity. This was possible because of how memo

This is called piggy-backing and it has several uses. Early PCs would stack/piggy-back memory chips to increase capacity. This was possible because of how memory was addressed. Another use is to find dead chips. If you put a working chip on top of a dead chip it will read the signals and work. Finally, in the case of the Vampire and other accelerators for PLCC socketed chips, the accelerator hi-jacks the bus telling the original CPU to do nothing while it takes over the bus. Electrically there is no trickery, think of each pin as plugging two cords into a power strip. Both get power even though there is one source. This probably would not work with modern chip packaging and modern frequencies, but it was a common technique back in the day.