Having asked my instructor about bong and tan sau while learning the SLT set, the answer was " for blocking high and blocking low incoming strikes. (wox on wox off muscle memory)I asked why the repetitious fook sau sequence and was told that, within the lineage it is known as "Little cannon " and is used to repel (center) opponent that is pressing in way too close, allowing you to free up the extended arm for a (wrist rotation) palm or fist strike for good measure (since you have them at your doorstep).