The Wing Chun Bong sau is a technique used to deflect a straight line force. The Wing hand is a very versatile technique, but it is also a weak position. Versatility comes from the fact that it can be changed into many other techniques. For example, the Wing hand, can be changed to a grabbing hand, a chopping hand, a thrusting hand, a palm up hand, a punch and more.
TIP: avoid the use of the Bong Sau to handle the initial clash
It is easy to get into the habit of overusing the Wing hand. Inherently, it is a weak structural technique which should not be used to hold back a force, nor to handle the initial clash with the opponent. Use a Bil sau instead. There are two types of Bong sau, one has the wrist straight, while the other has it bent.
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The Bong sau is not used to handle curved line forces. Curved forces such as the wide hook punch can be handled in various other ways. If you are strong relative to your opponent, then you can apply an inside Lan sau or Gan sau to clash directly with the force. If you are weaker than your opponent, then evade the force and hit. You can also use an outside Man sau or an outside Bil sau to intercept the force. Kicking is also appropriate in this situation.
The Wing Chun Kung Fu Digital Library