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. 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. The Bong sau is not used to handle curved line forces.