Here is a really rough explanation. Certainly it is not the only one. Quantum tunneling is to energy what 'empty' space is to mass. On earth, friction with our atmosphere slows a vehicle down. In space there is no real friction so a vehicle can accelerate much faster and maintain that velocity with a fraction of the energy it would need if it were in our atmosphere. The result is a greater maximum velocity.
Quantum tunneling is like reduced friction only for energy. It is theorized that the universe and space itself is not empty, but filled with a Quantum 'Foam'. This foam is the constanly entropic state of all energies that have, do, and will exist. Not unlike a celestial atmosphere. Any energy passing through it adheres to a specific wavelength that limits its maximum velocity. Imagine a billards table completely filled with balls, then take a few out so that all the balls can move, but restrictively. That is the quantum foam. As a photon tries to pass through the foam, it must move around the balls. If you trace the straightest path, it would look like a wave. Up around one, down around another, etc. However, because the balls are also moving, there occur, however briefly and rarely, moments when there is a straight path to the other side. With the balls, or quantum particles, out of the way, the light has no 'atmosphere' or 'friction' to keep it's speed in check. The photon's wavelenth is shortened exponentially, and the distance it travels is compressed, allowing it to reach the other side anywhere from 3 to 300 times its normal speed.
This not only occurs in space, but can happen through solid lead blocks as well. Scientists have transmitted data though a solid wall faster than it did through open air, hypothetically because small amounts of light were crossing through quantum tunnels and traveling faster than the beam of light that was unhindered. Many scientists have disputed such research though, so Quantum Tunneling is far from proven even by theorhetical mathematics.
Anyhow, just a short and sloppy explanation from one school of thought on the subject.
---
At least im still funny .....looking.
http://www.carrionfields.com