Larger capacitors act as inductors at AC frequencies. The larger the cap and the higher the frequency, the higher the "resistance" presented by the cap. This can impede or roll off frequency response (impedence). A small value cap across the large value cap allows those frequencies to pass. Kinda like overtaking a slower semi on a small road.
Bypassing Caps - what is it really about?
I understand the theory, but I've never been clear on the practice.
Some say its to extend the highs, but I see people using stuffy vintage caps as their bypass. I've also seen people bypass incredibly good existing caps with more, like Dueland on top of Dueland. So what is this really about? Is it about mixing tones of the capacitors?