Im not sure if by the 'shadow' card, you are refering to the 'base' card or the bottom card of the deck, prior to the laying down of the spread. I think I have seen this card refered to as this, so I just wanted to say - I have used this method in reading for many years and have always seen it as the 'Base' card and not the Shadow card.
IMHO - The 'Base' card refers and can be interpreted relatively to the reading as a 'basic underlying significant consideration which has implications relevant to the reading - a base, structure or foundation which the reading can be springboarded from'. You can do a reading just from the Base card. The base card is also something which is relevant to the client (and the seeker/client usually knows about the significance of the base card in a reading when its mentioned), but for the reader it can give much insight to the spread.
The Shadow card to me, suggests something which is hidden, either from the seeker or the reader, and may seemingly imply something secret, negative or greyed, or hidden. But the base card is very enlightening to the seeker and the reader. Maybe the Shadow card is something quite different
If you pick up a deck, you might like to get into the habit of looking at the base card before you do anything with the deck at all - before shuffling or rearranging the cards. I find it says something - and often about the next person or reading you will do with the deck
Go figure!! I do this with every deck, and have come to my own conclusions about what it is saying - maybe it will reveal something different to you
This is only my two cents worth, and if the base card is refered to the shadow card well so be it, but if its something entirely different then hey - thats the round about meaning of the Base card for you! LOL!!
Hope it helps, otherwise I have created a lot of confusion LOL!!