I don't know if i'd say trapping a good striker is easier than grabbing. I suppose there are many variables and context to consider. Perhaps pinning an arm that is not moving isn't too difficult but deflecting an incoming attack requires a lot more skill. I'm not too keen on pinning the arm either, a good striker will have good footwork and simply step back or fire the other hand at you. Now a quick smother or cover to setup a strike is one thing. But maybe it's the terminology that's the issue here. I think of a pin as something a little more substantial than a trap. This is where discussion gets hairy because we each have our own idea of what a term means and there's probably no standard definition. The best we can do is get a feel for how the majority of people interpret the term. Either way though pinning, trapping, smothering or covering an arm is a quick movement that doesn't involve a grip, the opponent is free to resist range out or strike with another tool which a good striker will certainly do. I think grabbing a good striker is more likely, with a grab you can still rush in eat a shot and get a grip on the guy. Despite counter arguments, to my knowledge we have yet to see a pure striker with no grappling background successfully resist a clinch or some kind of tie up in the UFC.
This is where understanding the principles of Wing Chun is essential, I believe. The quote is in my signature, but I'll repeat it here and use it to address some of your concerns:
"Intercept what comes; pursue what departs; when the hands are freed of obstructions, [they] attack instinctively."
This is how WC, and, I would argue, by extension, JKD, should be applied. Any "technique" is useless without an understanding of its purpose -- not only when and how it should be applied, but what it is expressing, and what it is trying to accomplish. So, with that in mind:
...the opponent is free to resist range out or strike with another tool which a good striker will certainly do.
Absolutely. That will be a striker's natural reaction.
If your opponent resists, yield and allow him to open his centerline for your next hand to attack.
If he tries to range out, follow after. You should have hit him already, simultaneously with your deflection, but regardless, you want to maintain forward pressure at all times.
If he launches the other hand, simply cover that line with your free hand, while your controlling hand senses the opening (created as he retracts his hand that was pinned) and follows in with an attack. Alternatively, just go straight in with your free hand and beat him to the punch, or step offline and cut it off from the outside, depending on how the pressure flows and what line he comes in on.
None of these "techniques" are in and of themselves an end goal; they're merely a means to an end, and must adhere to certain principles to be useful. If the arm you've pinned retracts, it's created an opening, and the pinning hand should immediately sense that and shoot in. If it tries to push you off center, you should let it go, sense the opening, and follow in with another attack. And you should always be ready to cut off the rear hand; looking for the next line. If your opponent tries to retreat, you must be able to sense that and follow in simultaneously. All of that needs to happen instantaneously, and instinctively as a result of your training. It's something that you really have to train to be able to apply, because much of it runs counter to what people want to do instinctively in sparring. It's also a comparatively more aggressive, and therefore a somewhat riskier method of fighting, so I think that you have to be willing to eat a lot of punches in the process of learning to make it work under stress, and against skilled opponents in a sparring context who are less aggressive and more careful than what you'd encounter in earnest combat or self-defense.
I can't say I'm there yet. I still have a lot more punches to eat myself before I can say that I can apply it with a high degree of success in a sparring context. But, even given my limited experience, I find that I'm still able to apply what I learn fairly well when I'm simply focused on, well... applying the fundamental principles of the art. For me, sparring is just a challenge; a way to experiment and see how I can apply things under pressure. And as long as I approach it that way, I tend to meet with decent results that, overall, validate my training. I don't approach it as a competitive venue. If I did, I don't think I would learn as much about the art that I practice. I might become a good fighter who learns to hit and not get hit, but with a narrower method, and to the exclusion of much that my art has to offer.
