How to Perform the "count on it" card trick

Andy in this video explains us how to perform the "count on it" card trick. First of all it involves with him taking a pack of cards. He asks the person who is about to witness the trick to take a certain amount of cards and asks the person to count the number. After the person has done it. He is asked to see the card corresponding to the count he has when Andy show him one by one. After the person is done with it there here comes the trick. Andy counts the card from one to king. That is counting one he takes 3 cards and places it at the bottom. And he proceeds the same way till king. And surprisingly the card that comes on the top of the pack is the same what the person had it in the counted position. Unbelievable believe it.