Cards shuffling and the meaning of "upside down" cards
In the process of tarot reading it usually depends how the cards are shuffled. The way of shuffling determines the probability of a card in "upside down" position. If you shuffle the cards with both hands, and you split the packages and rotate them against each other, the probability of "upside down" cards is up to 50%. However, if you do not turn the package over and just fold up the cards from the bottom, the incidence can be nearly zero.
The meaning of "upside down" cards
The meaning attributed to these "upside down" cards is very different. According to some interpretations these cards have the opposite (“shadow”) meaning of the original card. According to others, they have the same meaning as cards in the correct position.
Considering the fact, that the quantity of "upside down" cards can be significantly influenced by the way of shuffling, it depends on everybody, whether he decides intentionally to prefer tarot reading with "upside down" cards or not.