In Buah Pukul Tunggal 2 we see that the sweeping strike is done from outside. The shift of leg must be very fast, even before the opponent has put his step to the ground while throwing the punch. The pics below are just still illustrations.
ZR (left) throws a forward step punch
This time NZ swiftly shifts his right leg to the outer side of ZR's right leg to hook it back to his standing position. He chucks his left hand towards ZR's left neck slapping it to the opposite direction while his right hand diverts ZR's punch.
The after effect pose in reverse angle.