[QUOTE=SavvySavage;1125223]If the specific skills such as threading only work when the other guy is holding his arm out, like in chi sao, does that mean chi sao type drills don’t need to be trained at all?[/QUOTE]
Chi-sao is misunderstood, it is simply a method of using each others striking arms to try and exchange force through each others forearms to each others body.
We develop strong striking connections to our stance , hips, core, shoulders, elbows, wrists…each arm develops force forwards without retraction prior to delivery.
Each arm develops the ability to stab into a position like a double edged sword.
We give either edge of this sword a name for training it tan edge/ jum edge.
Each arm has the ability to be this sword, depending what side the arm is leading and what gate the opponent is moving into , relative to the center line, gives a direction to nearest weapon to nearest target.
Now if you stab something or drill or hammer a nail , if the alignment of forces arent right you dont maximize your force potential , you bend the nail, break the knife , etc…
If you align this section of force potential with the elbow ,gradually, dan chi-sao…then add each arm stabbing and wielding a shield [bong] lok sao, then stepping and wielding the shield/parry/bong etc…with stabbing..seung ma toi ma stepping stabbing …then angling off a charging stabbing attack, while stabbing back with the right edge of your own sword, in a strong stance to give force to the strike, balanced, not over turning to be stabbed yourself…etc.
you see a progressive idea, based on simple stab and kill while simultaneously defending too. Either with shield of bong, pak, jut, lop…or with the simple edge of the leading sword stab…tan or jum.
Many miss the basic stabbing ability of the arms and instead teach the sword not to stab, but to engage the other sword, or over shield/bong/pak, etc.., over control the sword with sword.This over controlling is becasue they are learning to duel sword to sword from the beginning. They stay in dan chi distances that are redundant to impact/stabbing/force exchanges and cant really reach properly, so they compensate by using ths tip of the sword moving off line, losing alignment…
This is due to the swords NOT exchanging stabbing force with a mutual goal, but now it is a way of dueling, competing , and the stabbing development is lost to sword chasing, feeling the swords, losing the body weight , and idea altogether. This is commonplace now.
In fighting we are able to simply face and stab with force to penetrate , not touch air, not so close we cant generate required hammer to nail head distance …
follow ?
we see the opponent also trying to make distances to use force and stab us too, the direction of their sword and body momentum relative to our centerline and gates, which sword is leading to flank etc…it gets complicated to say all at once without doing it, but easy when given systematic stages to work on and chain them together…
So you can see that you wont stab in a fight as you train to stab alternatively in a drill using each hand in rotation while standing in a basic stance.
Now when we fight we use one leading sword and one rear , man sao/wu sao.
If the opponent parries the leading stab , we simply stab straight in with the rear one .
If they parry onto our line of attack we use the leading sword to clear a path for the rear.
this is done also in chi-sao, so the parries and line decisions are mindless, and you simply let the opponent show you the next cut.
Stabbing a guy who gives you a clear opening ? or play sword games ?
the chi-sao also takes ona role play for mutual benefit, so the face off is equal in the drill each arm capable of stepping and stabbing , so we become natural at reacting to the right positions, balanced with force , timing, strike etc…then start again and randomly do something from the starting point of chi-sao drills. Stepping in, angling off side…check stances correct ones mistakes…
Later we add drills from attacking stances and leading arms, no rolling of chi-sao or attempts to , just quick entry to stab , parry or not , repeat
perfect …
Sensitivity is a BY-PRODUCT of chi-sao, not the goal.
You can also see that swords moving like windshield wipers in defense offer openings to stabbers. Why we dont use lateral blocking actions that cross over our own centerlines…we know whats coming at us in the same mistake 
center to center sword stab to sword stab , you overturn a little too much …