Block Physics is a curious little mod with drastic implications. It forces you to use sturdy blocks and stable construction for your structures to stay together. As a forewarning it’s strongly suggested to either back up your save or start a new one before loading this mod. You may end up destroying many of your favorite structures otherwise.
Blocks will stack normally but a good majority of them are not strong enough to hold up in mid air. As soon as you place them on the side of a block with no support below, they will fall to the ground.
However if you use stronger blocks like bricks or planks, blocks with more structure, you’ll be able to place them on the side of other less stable blocks.
It’s definitely a huge challenge to try and build things with this method in Minecraft. You won’t be able to easily construct things in mid-air and you’ll now need to plan every block you place much more carefully. Good luck because if you don’t things may come crashing down quick! And if you have structures already built, you may want to support them before they end up collapsing.