Actually, the join tool has been in the game since the first video but wasn't fully functional until now. You might have already seen it in one of the previous videos in the "Select Mode" category of the game menu.
As shown in the video the join tool can be used to connect bricks together. With that you can create buildings that are much more solid. That's very practical for cannons or defences in multiplayer. Furthermore there are a lot of new possibilities to build things (for example connect TNT-blocks to objects).
In detail, the join tool creates a connection between blocks that has a specific break-force. If the force of an impact is higher than that break-force, then the connection breaks. Stronger/heavier materials have a higher break force.
The reason why I implemented or finished the feature now is that I will start the work on the Inventor mode soon. Especially in this mode the tool will be very helpful because then you can create your own blocks by connecting some of them together and saving them.
Btw.: also check out the new facebook page of the game in the link section of the blog :)