Imagine a wave crest superimposed about twenty degrees out of parallel over a long, straight secondary ridge.   (This could be a standing wave, a traveling wave, or a harmonic… it hardly matters.)   Where the wave crest lies upwind of a slope, sink might be directed onto the hill, effectively canceling ridge lift.   Where the crest lies directly above a ridgeline, rotor could be a serious problem near the surface, yet the ridge may amplify the wave.   Then, where it lies slightly downwind of the hill ridge lift will almost certainly be enhanced – but the wave itself could be weakened, or even canceled.     Because each of these effects may be present at different points along the hill, it’s important to anticipate conditions at any particular place before committing oneself there.   Otherwise you might be rudely shot down.   Or… somewhere, you might find an opportunity to climb from low altitude right up into the wave.