Watching some of the pros play Hearthstone, I've noticed that they are able to immediately play a card returned to their hand, somehow issuing the instruction before the card rematerializes in their hand.
For example, the rope is burning and the pro has just calculated lethal. The pro does a flurry of activity which includes playing a Leeroy Jenkins, attacking with Leeroy Jenkins, using Shadowstep to return said Leeroy, playing Leeroy again, and attacking with the Leeroy Jenkins a second time. The rope burns out during the Shadowstep animation.
However, this is a pro, and after the rope burns out, the Leeroy Jenkins which is returning to hand, immediately comes back into play and attacks.
As an amateur, I have to wait until the minion becomes a card again and returns to my hand before I can play it the second time. I also have to wait until the card is in play to attack.
How do these pros do it?
This video, starting at 15:40 is a good example of a pro seemingly magically playing Leeroy Jenkins out of his hand without moving his mouse.
Best Answer
Trump is making use of the click-to-select then click-to-place mechanism in Hearthstone instead of the more usual drag process used.
When he plays Shadowstep, the card he targets to be returned will light up green as it's returned to your hand (assuming you have enough mana to play it) this means it's selectable, as your cursor is over the card (from selecting the Shadowstep target) it's easy to click again and "select" Leeroy.
Trump then clicks on the board just to the right of Leeroy as the card is being returned to the hand to issue the "place" command.
These two "events" (the Shadowstep click on Leeroy and the click on the board) are detected and queued up to be played in turn just the same as when you get ahead of the animations whilst dragging a lot of cards quickly around.