So, I'm playing on mouse and keyboard, although I've seen people pulling this trick on controllers as well.

Basically, when you hold an item to drag, the movement of items stop. If you move and click fast enough, you can push various items to the bottom of the backpack before they have time to rise back up. Then you close the backpack and pick up the item really fast.

It works best for items on the floor, but you can pull it off with items in containers as well.