This design is fairly simple. When you press the button, a green beam flashes. It works by digging a hole in the ground and putting an end gateway (minecraft:end_gateway, the thing that takes you to the second end, where you get elytra) at the bottom. Then, you put a dropper at the top facing down and fill it with dirt. Put a block on top and put a button on it. When you drop dirt in an end gateway, a beam flashes. The end gateway only actually takes you to the second end if you are in the first end.