Tripwire Hook

Tripwire hooks are a type of switch that can be placed at opposite ends of each other, connected by tripwire, for entities to activate. They are activated by most entities colliding with the tripwire that connects two tripwire hooks together (e.g. players, dropped items, some fired arrows and most mobs). However, thrown potions, some arrows, thrown Ender pearls, and thrown eyes of Ender do not activate tripwire hooks. Destroying the tripwire while not holding shears triggers a short redstone pulse.

Usage
The player first attaches hooks across from each other at most 40 blocks apart. Then, the player must place string across the gap between them. The string will be stretched across this gap once every block in between these hooks has been placed not requiring blocks beneath it to be placed. Once connected, the hooks will lower. At this point, walking on, or destroying the string (without shears) will cause redstone current from both hooks. The string must lie in a straight line between the two hooks, or else it will not activate when an entity collides with it. The pulse generated from the tripwire hook when activated can activate all redstone objects for a short time such as dispensers, often filled with arrows, and note blocks.

The tripwire will send a pulse of powered redstone, similar to pressing a button or standing on a pressure plate.

A mob or player of a great enough height to be partly inside a block will trigger the wire in that block, therefore they can also trigger a tripwire while sitting inside a minecart, with the tripwire being placed one block above the track. The string cannot be placed in the same block as the track. This can be utilized in conjunction with detector rails to detect if a passing cart has a player in it. (Assuming there are no mobs riding around, of course!) This greatly simplifies previous methods for doing this, and allows for easy cart-return mechanisms to prevent empty carts travelling around a minecart system.

The same can also be done with tripwires of varying heights and a pressure plate to detect something walking under them, thus allowing detection of the height of any mob (to the nearest block) walking through a doorway.

History
On May 28th, 2012, Dinnerbone began tweeting of a new idea he had. He began posting pictures and eventually released a video on tripwires. Before this, the player could not place string on the ground. Dinnerbone credited the idea to his fiancee MissMarzenia.

Trivia

 * Tripwire hooks, as well as the tripwire, can be "washed away" by water. If the tripwire is washed away first, the trap will act as if it was broken normally, without shears, and sends a charge before becoming inactive.
 * In terms of Redstone commuting, Tripwires behave just like Levers. This means that the block the Tripwire Hook is attached to will become a "Redstone block," which means the block will commute currents to any receivers (TNT, Redstone Wire, Redstone Lamp, Etc.) For example, for TNT to be activated via Trip system, the TNT must be placed on any side, above, or below the block a tripwire hook is latched to. This is a very nice way to install TNT traps.
 * Cutting tripwire with shears will not cause a tripwire hook to produce a redstone current, but it will cause its state to change which can be detected with a BUD switch.