I also didn't understand this right away, but this is amazing! Loving this. <3
One thing I was having issues with too was I was dragging a 2-slot block down into the bottom, and it kept snapping back into the grid.
It would be a nice UX enhancement that if I pulled a card to the very last slot, it would fill the slot above it if it's 2-slots. :) Just to make the interaction a little easier so it's not so precise.