The first RCX climber was slow and unbalanced. The second NXT version was much faster, but still not very stable since it used the same techniques. This third version is a new design, made so that it will automatically balance itself on one axis. Balance on the other axis is achieved with an acceleration sensor.
Imagine a line from the left wheels to the right wheels, through the middle of the robot. On this line, the robot tilts when it gets unbalanced. Unlike in the older versions, the main weight is now below this line. This means that it will keep itself balanced without help.
However, there is another possibility for the climber to crash. That happens when either of the sides loses a bit of grip or if both sides do not go as fast as each other. You can correct this error by making one of the sides go slower, so it will achieve balance again. You would only have to know how big the error is, and in what direction. This is what the Mindsensors Acceleration sensor measures. A relatively simple NXT-G program keeps track of the sensor and motor speeds. It goes up until it sees the ceiling and then comes down again.