Nice question, Warrior!
When you enter a battle the 8 cards you selected on your deck are shuffled and picked on a starting hand of 4.
The other cards are put on a queue to be drawn to your hand.
Every time you deploy a card, it is put at the end of the queue and after a few seconds another card will be drawn to your hand.
Now it's up to your skills to make the best use of them.
A good idea is to keep trying different strategies until you find the best attack and defense balance.
See you on the battlefield! ;)
Over