1) Going round the opponent with something like a Nine Palace Step. Unlike in flawed application training, an attacker will not pick one vector and stay on it. He or she will swivel quite naturally to find and hit you wherever you may go, unless you really are extremely fleet of foot. If they are at the centre, they only have to turn a few degrees, whilst you have to travel a couple of feet.
2) Any kind of throw. You see this in competitions, where people shoot in for the Double Hand Takes Legs, or they jump and swivel in for a White Crane Flaps Its Wings. Normally they expend energy and give away position for very little effect.
3) Effective striking, by which I mean striking from a position where it's hard for the opponent to strike back. If you are just standing centre line to centre line, exchanging punches, you are not using your tai chi chuan to its best advantage.
4) Sweeping to take their balance. Again, in competitions particularly one sees major shin-kicking contests where the opponents try to apply sweeps and trips with little effect (though a good kick to the shin can have its very own special effect...)
5) Locking an arm, or controlling the motion of the opponent.
There are probably more, but these are the most obvious to me.
Lots of the syllabus in tai chi chuan, and I imagine, in other martial arts, is based on a kind of end result: techniques applied once you have already done a certain amount of work on the opponent. I'm not sure if I want to specify exactly what this might be at this time, mainly because I can't think of a way of explaining it that isn't misleading. So I'll ruminate a while longer, and get back to you.
