I'm sorry, I don't know about the weather shield; I suppose I've been out of the market too long.
To weave the valley; lay the shingles in single horizontal rows, let the right shingles overlap the valley by 1' with no nails within 1' of the valley on either side, now let the left shingle row overlap the valley by 1' again without any nails within 1' of the center of the valley, now right run back over, left run back over and so on. Now the center of the valley is flexible and at least 4 layers thick.
With the California valley, the water comes running down the side that is under and wants to continue in that direction which makes it go under the side that is on top, thus you have a leak. When the valley leaks it is extremely difficult to find the leak because it may be several feet above where it runs through, so you end up coating the entire valley and hoping you have hit the spot that is leaking.
Take a drive around town looking at valleys, you will see what I mean.