well you have to remain at striking distance or the technique you hope to strike with is ineffective.. ultimately trapping is still restricted to striking distance.. it has to be by definition? how ever you cut it you must not fall short or move in to close..
A. Yes, trapping and striking as I've described are only useful in certain circumstances (i.e. you do it whilst moving towards them at the same time, and you have to be fairly close to start with). When I edited my post, I gave an example where you don't have to be square on (slipping a punch, moving in and doing an arm bar across your chest), but you do still have to be close to them (and move in closer) to pull it off.
why is this rare in chi sau? you do not often see one guy out positioned?
A. Being square on allows trapping to occur so often because being square on (facing their centre-line) means that your opponent is in range of both hands.
WC practitioners mostly like to keep the opponent in range of both hands.
It can be done when not face to face, e.g. you can be 'square on' facing their centreline and just trap the arm nearest you.