You can find a detailed explanation I made on video here:
Wing Chun is a martial art based on concepts and principles. Economy of motion and simplicity is paramount. This post will be about the Four Fundamental Principles of Wing Chun.
Number one:
If the way is free, go forward!
When there is nothing in the way, you simply attack with a step and punch. If the opponent is open, there is no technique to do; you just hit him. Attacking your opponent will cause a few finite number of possible outcomes. One, he will do nothing and get hit with your punch. Two, he will move out of the way, into kicking range, and you will kick him. Three, he will try to block your punch, in which case you were now successful in making contact with the opponent and can use the main Wing Chun skill set of Chi-Sau.
Number two:
If the way is obstructed, stick to what you contact with forward pressure!
If outcome number 3 happens, then the way was obstructed and you stick to him like glue. From here you can feel what he's doing. This sensitivity allows you to instantly apply a tailor made response to the situation. You don't know what your "technique" is going to be until you make contact, and it's your opponent that tells you how to defeat him. If YOU don't even know what your response will be, then there is no way for your opponent to either. Remember, he doesn't know that you know Wing Chun, and this element of surprise is in your favor.
Number three:
Give way to greater force!
Once you have contact with the opponent, you will know whether he is stronger than you or not. If he has greater force, then you don't fight him. Yield the right of way to him and strike him as he goes by you. This is certainly something that has to be felt in person to really understand. Even on video, it doesn't convey the extreme effect this has on the opponent.
Number four:
If the opponent withdraws or retreats, follow through!
If you were successful at closing the distance, then you don't allow the opponent to get his distance again. If you hit him hard enough that it sends him away from you, then you stay with him to continue your attack. If he's creating space, you want to make sure he doesn't have the chance to retrieve a weapon, whether from his pocket or from the environment around him, let alone seeking help from friends.
This was just a quick overview of a few of the concepts of Wing Chun. Each of these have too much depth to them to go into in a short blog.
-Sifu Matt