The confusion is deeper…stop asking about the fok sao in chi-sao…ask what am I developing ? why use a wrist in drills when I wont in fighting…why train to make contact with a tan sao using a vu-sao wristing deflection ?
the chi-sao is a stage by stage developmental path to free fighting with 2 free hands…nothing to do with the wrists 
The main culprit is when students are taught SLT, The beginning slow section has no jum sao, just a wristing Vu sao back then fook forwards, then Vu-sao back…common mistake made because the teacher didnt get the whole IDEA 
No Jum sao= inwards elbow/inwards forearm striking energy position for drilling later…
missing
iow..the tan goes out , turns to huen as the elbow contracts inwards to make an ENERGY = jum sao [you cant see this ENERGY / contraction , it has to be explained by someone who knows]. You contract the shoulder girdle to hold the elbow low and in while seeing only one finger in the hand…contraction held for 2 seconds , then the fingers come up and the elbow relaxes to make the VU-sao to come back …repeat 3 times for significance/importance…
IOW many MISS the jum sao altogether assuming it is a wristing action of the vu fingers pointing up from SLT…wrong…not the vusao, but the missing jum-sao..MISSING,MISSING,MISSING…
You need the JUM & TAN to develop into partners of energy…
JUM =inwards elbow ~ TAN sao = elbow spreads off the line
so regardless of what flank you attack from you have 2 arms in rotation each covering the line from the given side…iow one strike will use outside energy /alignment while striking to stop entry to you..the following strike will take over with inwards energy to close down the same side…
the tut sao or shaving hands teaches seamless line control , so nothing can enter the centerline…
why 2 energies from the same arms ? why have a strike that can both keep an elbow on the line and spread it off the line while each hits forwards ON THE LINE with little or no thought to the changing energy…?
The forearms act as deflection surfaces from the acute angle the inwards elbows create.
they utilize explosive force ‘ging’ and recover back to the centerline in fast repetitive strikes capable of changing the energy in each arm , relative to the positions and angles you and yur opponent take…motion, fighting for positions…
Your VT idea tactically is to deliver more attacking actions than the opponent in any given time period…so hitting is good but how can you get an edge on the guy when he also has 2 arms hitting you ? angling and using the extended striking arm in a 2/dual action strike= economy of motion…either arm can use the inwards or outwards energy to maintain line integrity , while striking out…facing and angling …the energy of the striking arms changing …you cant see this energy…you have to have it explained in the dan chi-sao…chi-sao so you dont make the mistake of adopting a wristing, feeling, controlling, hand chasing, waste of time
If the arm is intercepted we use the contact strike drills from our training…the arms are used to contact so they dont lose their elbow angles ability to stay on target…simple.
If you do use the wrists your TO FAR AWAY FROM THE TARGET TO HIT IT 
AND IMPOSSIBLE TO DEVELOP SIMULTANEOUS STRIKE DEFLECTIONS unless you always use 2 extended arms to fight the opponents 1…think about it, your training to use 2 hands constantly against one…? while standing in front of them so they can wail on you ? really 
with inwards/outwards elbow/forearm controlled strikes you can control entry to your self by simply ..angling to face…either side , move to gain angles allowing the idea to function… with either lead arm…while striking fast and not stopping to over trap because you lack the basic striking development stages…missing from your curriculum
completely…
Instead you try to make the system function with a tan that is thrown out to the shoulder, the outside upper gate..etc…to block while the striking arm is just a fist or a palm or a finger jab…iow you NEED 2 hands …always…because you lack a tactical idea you adopt the drill positions for the safety blanket of control you get that goes out the window asa you try free fighting 
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The common MISTAKE made by many students, is to use the wrists as force connections/feelers in a drill …this leads you to become a seeker of arms to feel and control before hitting…before hitting…before hitting…
Striking with arms capable of utilizing deflection & strike in the same beat is the aim…
EACH arm trained through the drills to encompass line closure/protection, while striking out to the target before them…in rotation, angling for attack angles and moving to face a moving target…
Sadly the drill has been so watered down by teachers with little or no contact to the source…
the wrist is isolated from the strike drills and not used as a deflection simply because it takes you striking arm away from the target…it is ingraining the wrists movement and force out at the end of the arm …creating a long lever that also crosses the strike/centerline, due to the nature of the wrists looking for the contact .
there are 2 stages of distance in chi-sao …one is the introduction of strikes using opposite forces daan chi sao…you are out of striking range..developing elbow energy and alignment …just like SLT positions..so you are using the form in isolated actions…jum versus tan energy..one is trying to displace the other while staying on line to the center…drilling a relationship only relative to YOU and your elbow on your line …for use when advancing to later stages involving motion and angling…so you dont lose you aim and go off target by simply touching the guys energy in his arms…
energy can be misinterpreted to be chi/qi for lack of relativity to VT..iow the energy we are developing is SPECIFIC to our goals …attacking etc…attack/defense simultaneously…tactical motion…
The Little Idea in SLT isnt how to develop your wrists feeling …fok is NO ENERGY in the elbow iow neither jum or tan energy…you cant have energy constantly in the elbow so fok is the ‘off switch’ in the drill to make our arms go limp but still hold the elbow centered without having tan or jum energy flowing into it…or recovering a extended strike back to neutral energy fok sao…
fok = no elbow energy for drilling…its wrist positions is redundant…in chi-sao the fook is resting on the forearm of the partner becasu you are now in contact distances…trying to both strike and use elbow positions together…in one striking action…jum elbow in strikes keeping tan attempt to make your elbow go out so it caan displace as it strikes …
one strike attacks another strike… strike v strike…no I strike responded by a block then a strike…ingrained a 10000 times…
If in chi-sao one of you is doing a tan attack and the other reponds with a wrist block its wrong …
you should be striking the attack with a counter strike…if not , where is the simultaneous action…
maybe its lost in structure gobeldygook…pointing tan to a shoulder while the wrist defelcts , to far to hit the guy tannin into you…over and over and over…so when are you going to develop hitting a guy stepping into you ? answer your not, your going to do all kinds of structure drills, fancy chi-sao stuff etc…
why because the idea has been made into various levels of $ making… stages/belts/grades/certificates : )