AC Circuit + Parallel resonant band-stop filter
Assuming you set node 0 as your "ground" or "reference" node, you only care about what goes in at node 1 (\$v_{in}\$) and what comes out at node 2 (\$v_{out}\$). The voltage at node 2 fully determines the voltage and current through \$R_{load}\$.

Maybe you should prove what's going on by turning the capacitor and inductor into complex impedances in parallel, then finding the voltage at node 2 via the resistor divider equation. You can plot this (the magnitude, not just the real part) graphically as a function of frequency and see the voltage fall off at a particular frequency.

Another thing to realize is that you can set your ground/reference node ANYWHERE, but looking at what we want to look at here, it's most convenient to place it at node 0. But everything still works out exactly the same if you've done it correctly.

Added based on question edit:

Imagine we break it up into this pair of DC circuits. Just because the voltage is reversed, it doesn't skip the circuit. It just goes around the other way.







simulate this circuit &ndash; Schematic created using CircuitLab