When you have a string line (up to 41 blocks long) connected to a hook only on west or south end, any update to the line (walking into/out of the string, breaking the string or placing string on the other end of the line) will create block updates next to the hook. It doesn't happen if the hook is facing west/south.
- Place tripwire hook so it faces north or east
- Place and open a trapdoor next to it
- Place a string in front of the hook
→ You will see that the trapdoor is updated and switches to the closed state