This is a final test in accuracy, speed and power, by doing something contrary to your human instinct.
Power break is a major part in conquering your fear and is a test of confidence using either your arm or your leg to break boards made from a variety of materials to demonstrate learnt techniques. The amount to be broken is related to size and ability at any specific level of the training.
Once you are autonomous with various techniques you should be able to use them with confidence and efficacy should you have to defend yourself in a street situation.