Core skills to be learned in chi sao by Victor Parlati
There are many. But one of them has to be coming in and pressuring your opponent's space. if you're just making a hit with no followup and control precisely because you haven't taken his space away - then you're inviting him to counter you by "leaving money on the table". (Similar in principle to what a good wrestler/grappler tries to do with a clinch).
So you can't afford to leave those openings when using wing chun. Because you're fighting very close - which means he can hit you back (or grab you) in a heartbeat if you're not diligent. So the system needs multiple unanswered hits achieved by controlling his space (and his balance)....so you must come in sharply and very aggressively once you control a line...
otherwise you're not much better than a boxer who hits with a jab or two and then backs out. He's inviting his opponent to recover and launch strikes/attacks of his own.
BUT WING CHUN KUNG FU IS A CLOSE RANGE INFIGHT...
that features short, quick, straight punches and other strikes that can be multipled if one is controlling whatever line he's working on...usually meaning one that leads to the vertical center of mass of your opponent (regardless of where his main centerline is facing).
So although we don't always followup on the same line as the first punch (ie.- the proverbial "chain punch attack")...nonetheless...you should be able to use something similar often enough if you've come into his space and have jammed him up.
Which is not to say that you never strike just once or twice on open lines (wherever they may be - at his center of mass or somewhere else)...at any given moment - because you do.
But one of the most important skills learned in chi sao is to TAKE CONTROL of a line (a path) that leads to hard targets on his body and PRESSURE FORWARD while attacking with multiple strikes on that line. Which means that your entire body structure is attacking that "point" on his body. Your legs are driving to that place, your hips, and of course your elbow and hand. So that there's follow through at impact with everything you've got - with your elbow and the bottom three knuckles of your (usually) vertical fist at the forefront. (or perhaps your palm or the ridge of your hand).
One of the most important CORE PRINCIPLES/SKILLS one learns during chi sao when training in wing chun.
Go in with your whole body and take his space away...the space he'll need to attack you or defend against you. Take control of it. You own it - and now you're going to force the issue because you've taken control of the line - and you've done so with your other skills developed in chi sao).
But if you're not taking his space away at every opportunity (and constantly looking to create such opportunities) - you're not doing wing chun. You're bouncing in-and-out of wing chun and something else (ie.- boxing) unnecessarily. But with these skills - which are primarily developed within chi sao practice - you can end a confrontation very quickly.
More to be said about this and other core principles later.