Well it basically works like this.
In a huge deck, you pack a ton of hourglasses.
So, when you draw your first, your draw speed doubles.
This effectively halves the size of your deck.
1 HG=30 cards
2 HG=20 cards
3 HG=15 cards
4 HG=12 cards
5 HG=10 cards
6 HG=~8.6 cards
Make sense?
So basically they let you rip through the deck, giving you what you want sooner than you might think. Drawing power in this game is equivalent to playing multiple turns, if you have enough towers to keep up.