This video provides a great example of how to escape from the back choke hold, also known as a rear naked choke.

As soon as you feel the arm coming around from behind you, it is very important to close your chin. Do not let this hand get around your neck by bypassing the chin.

After your chin is down and neck  protected, step to the side and grab their wrist at the same time.

Now you can strike to the groin to distract your opponent and loosen the choke grip. So, step to the side, grabbing the wrist, and hitting to the groin. Now as the opponent leans down, throw a sharp elbow up to the opponents head.

In the second scenario, it explains what to do if someone attempts to lift you off the ground with the back choke.

This makes it difficult to move to the side, so we take an alternative approach. First, as always, close your chin by leaning your head forward and using your chin to protect the neck. Grab your opponents arm with both hands. Now move one leg as if you where stepping, and with this same leg your going to kick to the opponents groin.
And those are two ways to escape from a back choke hold.