Tripwire Hook

Tripwire Hooks are non-solid blocks that will be added in Update 0.13.0. It can be used to detect entities when part of a tripwire circuit.

Obtaining
Tripwire hooks can be obtained as a junk item through the process of Fishing. They can also be obtained by crafting.

Crafting

 * 1 Wooden Plank + 1 Stick + 1 Iron Ingot => 2 Tripwire Hooks

Uses
Tripwire hooks can be placed on any opaque block, and must be connected to another tripwire hook through Tripwire (Placed String). When any entity walks through the tripwire, the tripwire hook will trigger a redstone signal.

Trivia

 * Tripwire itself will generate no redstone signal- it must be connected to a hook to do so.
 * Walking through or directly under the tripwire hook will not generate a redstone signal.
 * Tripwire hooks also generate naturally in Jungle Temples.
 * However, the latter has not been added to Minecraft PE yet.