Make yourself comfortable this is going to get deep , chi sau is a sophisticated tool indeed.
In order to understand how trapping is applied in Wing Chun one must first have a basic understanding of the theory and mechanics involved in chi sau.
Basically speaking , the theory of chi sau can be likened to that of an inflatable ball bobbing around in a swimming pool.
You try to apply pressure to the ball but it spins away and springs back up due to the water acting on it from underneath.
The resilient pressure of the water acting on the inflatable ball is the same as what we in Wing Chun call "springy force or "forward force"
In essence , we are always trying to project this sphere (our inflatable pool ball ) at the opponents centreline.
The revolving force of the sphere is enacted when the arms are in the position of Tan sau , Fook sau and Bong sau.
Any force contacting this sphere can be spun away and redirected exactly like the ball in the pool analogy.
If we are projecting our sphere properly and have a continuous flow of "forward force " our arms are positioned correctly , force focused properly to the centreline , and our force is projected evenly along our forearm from elbow to wrist , then nothing can get in.
How does this relate to trapping you might ask?
Well if someone is at a very high level and "rolling" correctly there is no trapping.
For the trap to be pulled off correctly , the other person has to be making a mistake , eg. uneven force along the forearm.
Chi sau teaches you to have evenly projected force along the whole forearm , if someone has no experience of chi sau there will be a weakness some where along the arm that can be exploited.
Other traps take advantage of incorrect positioning such as Bong sau wandering over the centreline , in which case the hands are crossed and now vulnerable to the opponent dropping his Fook sau and trapping both the hands.
Similarly if the persons Fook sau is wandering over the centreline , it is susceptible to being pinned by the opponents elbow (Bong sau).
Each particular trap is used for a particular error that the opponent is making , the skill is in having the sensitivity to realise which way his force is going and apply the specific trap for that situation.
The opportunity for the trap is either there or it isn't.
As I have said many times before , the best counter to trapping is correct rolling , don't make any mistakes and it is very hard for you to be trapped.
But by the same token , we are human beings , not machines , and as such we are apt to make mistakes .
That is why for every trap , there is a counter to that trap.
So in summary the opponent must be making a fundamental error for you to execute a trap , you don't just trap because you feel like it .
Most times there is no need to trap anyway , because they have so many weaknesses in their defence that you can just expand your angles and "crush" their structure or just hit through all the gaps.