From Wikimedia GIF created by Tomruen
As you can see, when it stops to be a triangle, it seems to be impossible. The truth is there is a jump between the purple and red cubes in the 3rd L(brown).
From the angle you are looking at, it seem for it to be connected to each other. That makes it seem impossible because you cant see the jump between the purple and red cubes in the 3rd L(brown).