The crash is an ideal response to any committed upper body striking attack, and will even work against a number of grabbing attacks. Unlike defense techniques that require matching a specific defense to a specific attack, the crash will work equally well against front hand and rear hand straight punches and hooks, including a wild barrage of strikes. This makes it an extremely effective default/conditioned response when you find yourself under attack.
The crash is simply an entry that disrupts your opponent's attack. It puts you in a position where you can easily gain control and begin your own blast until your opponent is out of commission. The best way to train the crash is to have your partner begin slowly with a single punching attack. Progress to where your partner can throw any punch or punches with full speed and power. Practice the crash over and over again until it becomes a conditioned response.
In the images below you'll see the crash being used against a right cross. There are other options for crashing, and different follow ups depending on where you end up after crashing. Experiment, and figure out what works best for you.
See more self defense techniques, or more from the Fundamental Five: