It's simple. You scrape for a product/product ID, if its loaded and in stock...you use the PID to type some code that adds to cart on a logged in account. The account and password is then shared, when you log into that account the cart is full with the item even though its never actually publicly touched the site.
A friend did it for me last week with a pair of Q54 AJ1's, I had them a week before the drop actually took place. This happens all the time. I think this shoe however never hit END and it was all trolling. I THINK.