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.