Are you asking about any given style's conceptual approach to combat, or are you talking about how "the most people tend to apply it in sparring," or are you asking about how any given individual (who may differ greatly from the majority - which, by the way, doesn't make them wrong; in fact, it often makes them exceptional) applies it in sparring, or are you asking about how it is applied in other contexts such as self-defense, either generally, or specifically?
I think you will find as wide a diversity of practitioners in any given style, with different understandings and interpretations of their art, different abilities, different personalities, different levels of experience, a different focus, and very different results, regardless of which context you choose.
As for myself, I can't claim to be that experienced, but I do practice sparring against different styles and applying the principles of my art (Wing Chun). In my opinion, a lot of people get caught up in technique land, or take a "chisao" mentality to a fight, and are ineffective because of it.
Wing Chun is based on two principles, and really, the second is just a means to the end of making good on the first. The first one is centerline theory; that is, to attack and defend on the centerline between you and the opponent, as this is both the most direct route, and the one that protects you - or else leaves you very close to where you need to be to protect yourself.
The second, is the maxim in my signature: “"Intercept what comes; pursue what departs; when the hands are freed of obstructions, [they] attack instinctively”." What this means, essentially, is that we stay close, and fill any gap that appears. We don't want to allow the opponent to range out, because the principles we practice work best close in where our hands can remain an immediate threat, and we can use the basic structures and hand "techniques" that we practice to best effect. This concept is identical to fencing, and you might aptly describe Wing Chun as "fencing with fists."
I take a very simple approach in sparring. I don't think about anything other than delivering a good attack on the centerline, preferably on a line that covers me from the most immediate threat. But I ensure that I'm always attacking, and that I'm occupying the centerline - and if anything pushes me off from it, I yield and strike with the next hand the moment I detect that the center is open, or going to be open again. I'm not thinking about chisao. I'm not chasing hands or trying to get a "bridge." I'm just doing my part to offend the opponent, follow in, and let whatever "techniques" I need flow naturally from there, if they're needed at all. As far as timing is concerned, I do try to catch my opponent simultaneously in his actions, employing something akin to a "counter punching" method in boxing, as Wing Chun is particularly well suited for that.
