bongs are all the same on dummy and the system …just trained on the low arm for jing [displacement force] slap !! to remove laterally any arm that has xed ‘spatialy’ your line to the target..in other words it doesnt need contact [wrong chisao thinking] to ballisticly remove an arm as we attack…remember we are attacking never suddenly recieveng a guys energy to borrow bs…thats why the chisao has your tan [ 1/2 hit] bridged by a fook..simply a role for each to develop the correct lateral displacement of the bong as it raises the elbow and X 's the centerline ..then back to elbow in striking…its not a fighting application to attack with a series of arm blocks ,as some see it …
In Chum kil we marry the vu sao to the bong becasue itis the way to recover a bridged attacking arm..and immediatley strike [ not lop] from the vusao as the bong sao elbow drops ..the action /sequence is tutsao…simply to attack without stopping for an arm over ours…
The pole teaches this ballistic displacment force , so powerful and shocking …a lop sao is only required [as bilgee] if the bong didnt work as it was supposed to…
you empower your bong to be a force unto itself not a turning away , passive chi-sao misinterpretation …that doesnt work against unwilling fighters …
simply 'jamming ’ with it as so many teach is making it not function as it was designed, the ‘jam’ is lan sao/fence.
double low bongs in the last section of chum kilsimply teach us that we can attack either lkeg leading from a side and use either bong…we do both low to develop inward ‘slapping’ left to right & vice versa and not damage our necks by constant upper lat contractions on our necks…
the bong doesnt go up and down in chi-sao …it goes so the fingers , once in a raised position, point to your parnters bicep…his fook is on line controlling the force at his elbow, if he applies force at his wrist it will be taken offline , naturally , with the motion of bong sao , …inan unthinking attack IF an arm bridges ours we regaing the attack with the rear vusao/strike as the bong opens the line right to left or vice versa…not for sticking …once the arm is moved off the line bong isnt needed becasue the rear vu/strike takes up the line transfer protocol of the system…seamlessly maintaining an unbroken center while we keep blitizing the guy…not many know this method…
the redundancy factor of chi-sao is simple, we never [ shouldnt ;)] attack with 2 equally extnded arms ..rather one leading strike and a rear hand/strike..only turning to pratner displacement line clearing protocol if an obstruction is met…
infront, pak …contact under , jut…arm over our attack line bong vu/strike,…bong doesnt wok or the jut feels an impenetrable arm position , lop on the axis of the guys body , maintain attack again…
all done in the blink of an eye 
all bongs are the same just the levels have changed to allow certain things …