The video posted above shows Day[9] battling it out with none other than Rob Simpson. Specifically, what we'll be seeing is how important the concept of "topdecking" is. So, what does it mean? Basically, it means drawing the the right card at the right time. Perhaps your opponent is down to three life points, and you draw a Lightning Strike on your turn. That's a topdeck.
Topdecking also doesn't mean drawing the card that will immediately kill your opponent, as it can mean drawing the card that will give you the utmost advantage. For example, drawing a Damnation when your opponent has played a ton of creatures.
As for Hearthstone it would be getting an Ysera when you're down to one life point, or drawing a Pyroblast when your opponent is down to ten life points.
