This is a bold method for performing the Card Under Box trick. The card load happens right before the card is revealed to be under the box, which makes this a versatile method and ideal for off the cuff situations.
Intermediate - Advanced
I first saw Bill Malone do this in one of his On The Loose DVD's. I'm sure it's much older than that but I'm not exactly sure where else it has been published. Let me know if the comments if you have any more info on this method.