So this video was meant to show you how to use the alpha trigger in GD. Basically what you do is you select a block and then place it on the ground. Then you go to the tab where the alpha trigger is. (shown in video) After this you make the alpha trigger whatever group ID is available and then de-select the alpha trigger. Next you select the block you put down and add it to the same group ID as the alpha trigger.
(BE SURE TO MAKE THE OPACITY AND FADETIME 0 WHEN USING THE ALPHA TRIGGER)