Greetings.
It is important to note that in boxing, the focus is offense and defense comes mainly from evasive footwork and body movements.
Wing Chun focuses on stable structure and NOT committing fully to every strike as to maintain stability and control posture.
So power is generated in different ways and TRANSFERRED differently too.
So in Wing Chun, energy transfer is very important. This is what is meant by good biomechanics. Also, being a good athlete helps.
Yet in Wing Chun, the idea is to beat athleticism with skill. So the skill serves as an equalizer against physically "superior" foes.
Unfortunately, good biomechanics can be taught only by qualified teachers that know about it. There are not that many, unfortunately.
Also, it takes a dedicated student to learn these things. Again, not as common.
Normally, Wing Chun power is felt when you see a practitioner move effortlessly and you feel an unexpected explosion that trembles you whole body no matter where you are hit...
Much fun!
Hope that helps.
Juan M. Mercado