Does decoupling capacitors try to charge the fluctuating source back?
Generally speaking, it isn't the source that fluctuates, it's the load. One common example is digital circuits that draw short-term spikes of current when their outputs switch. And the best way to think about it is that there is some significant impedance between the source and the decoupling cap, especially at higher frequencies.



simulate this circuit &ndash; Schematic created using CircuitLab
So while the DC signal flows from the source to the load, the high-frequency AC noise imposed by the load flows mostly through the capacitor, both charging and discharging it. It &quot;decouples&quot; the noise from flowing through the power supply impedance, shorting it to ground instead. The goal is to keep the green noise loop as short as possible.
The peak noise current is never greater than the DC current, so there is never a net flow &quot;backwards&quot; through the power supply.