The correct technique is to for the man to stand with the full weight on his right foot. The left foot should then be swept back quickly and crossed behind the right foot in promenade position into whisk. With the weight now evenly distributed between the right and left foot, the man should be standing on the flat of the right foot and the ball of the left foot. The lady of course stands on her left foot and sweeps her right foot back in promenade position at the same time turning her head sharply to her right.
Here's a nice tango routine incorporating the whisk. Dance two walks followed by an oversway with a timing of S S Q Q S S. Next do a chasse to right, RLR and cross left foot behind right foot in promenade position into whisk. Then step right foot forward in PP and CBMP and place left foot forward in PP without weight. The timing is Q & Q S Q Q or Q & Q Q Q S but I personally prefer the first timing. Finish with a closed promenade, S Q Q S. Happy dancing!