The trigger block is used to activate other special blocks like the conveyor-belt, TNT or the accelerator. The trigger has a transparent field that reacts to other objects. If an object enters the field, the trigger sends out a signal that activates all connected objects. When the object leaves the field, all connected blocks are being deactivated.
Blocks can be connected to the trigger by pressing the "Connect" button in the menu which enters a new game-mode. After clicking the button you can select the objects that should be activated. To know which objects are connected to a trigger, there are arrows between them (They only show up if the trigger is selected in the management-mode).
Another feature is that you can set a delay. This is the time between a block entering the trigger-field and the activation of the connected blocks. Furthermore the trigger can be activated or deactivated (like other energy-consuming blocks).
As shown in the video this enables you the build more powerful and more dynamic inventions (automatic cannons,...). I hope you like the new block as much as I do, I already spent hours on building and optimizing my own crazy machines (I unfortunately lost them due to tests with the save/load mechanism with the trigger).
So what's going to be in the game next? The next update will also be a new block which makes the trigger much more powerful and allows you to build more complex circuits.