> How does effect knows that it needs to call this lambda whenever parity gets changed?

The call `parity.get()` will register a dependency on the function that is passed to `effect()`. When `parity` is updated, the function is called.

> Will it call this lambda on any signal change?

Only when a dependent signal changes.

In this case, `parity` depends on `isEven` and `isEven` depends on `counter`. So when `counter` is updated, that whole dependency chain is invalidated leading to `parity` getting invalidated, and that callback re-running.