You are watching a ship being loaded with large crates. The ship is held in place by several long steel cables attached to the dock. At one point, a crate bumps one of the cables and sends a wave pulse along the cable. The pulse moves rather slowly up to the bow of the ship. A few minutes later another crate strikes the same cable, and this time the wave pulse shoots up along the cable very quickly, reaching the bow almost faster than your eyes can follow it. Explain how this is possible.