This design appears to be broken in 1.9. It might not be fixable, but I am not sure if it is or not.

This is a very secret redstone input. How you use it, is you take a fishing rod, and land the hook in the middle of a floor, then quickly pull back (you don't nessisairly have to pull it back quickly, but it is more reliable if you do that), and a redstone signal is outputted. Note that the floor, at least the part that is used as the secret input has to be made of transparent blocks, such as slabs. Also, if the floor is possible to hit with the fishing rod from higher, it is possible to break the input by pulling the iron golem completely out of the chamber.

The only 2 things that would give it away (and it is unlikely this would happen) is:
1. If you highlght the blocks where the fishing rod would touch,
sometimes,
the block will not be selected anymore, and punching.would do nothing.
However, it is unlikely someone would notice this, and if they did
notice, they probably wouldn't know what is going on.
2. There might
be some quiet and occasional foot step noises, but even if someone hears
it, they likely won't assume there is a secret input nearby.

How it works, is that under the floor, there is an iron golem, who is in the floor, in a chamber. To get an iron golem inside the floor, you can push the floor blocks in place with pistons after the iron golem is in the chamber. On the floor of the chamber, there is a pressure plate, with an inverted output. When you hit, and pull the iron golem up, he will no longer be on the pressure plate.