From a torch key to a BUD switch, a hidden input is a hidden way to trigger a contraption in Minecraft redstone. Follow these steps for one way to create a hidden input:
Build a hidden input is to start with 4 blocks in a row.
Place a sticky piston on top of the left block in the row, and next to that, place a redstone block.
Place another block on top of the sticky piston, and then place a sticky piston on top of the redstone block.
The redstone block powers the sticky piston, pushing the piston outward to the right if placed correctly.
Place a redstone block on the extended sticky piston, and then place a block on top of the base part of the extended sticky piston.
Then place a sticky piston facing toward the block you just placed.
Place the items on which you want the contraption to operate on the right side of the contraption.
For the example shown, place a repeater on the far right block, which is connected to a glowstone. To operate, put a redstone torch on the left corner of the contraption on the wall.
In the example, that step powers the sticky piston on the bottom, extending the redstone block so that it touches the repeater, which then briefly lights up the glowstone block. Then the other sticky pistons are also activated, and the mechanism resets itself by waiting for a new torch to reactivate it.