When you Set Up A Tripwire Hook And Tripwire Above The Ground(Floating In Mid Air) And The Hook Is Surrounded By Blocks Apart From The Tripwire's Block, When You Pass It, It Doesn't Activate Any Redstone Connected To It.

How To Find Out/Reproduce:

1.Set Up A Tripwire Above The Ground
2. Surround The Hook With Blocks(Apart From The Tripwire)
3. Hook It Up To Some Redstone(Maybe A Command Block (When Activated) Will Say Hi)
4. Pass By It And See Whether The Redstone Activates.

Check Out The Pictures Below.
The Command Block Will Say "HELLO MOJANG" When Activated.

Yeah... It Didn't Activate