This video illustrates tapping on the '3' or the upbeat (halfway through the beat). Count '1-2-3-4' for each beat. ('1' being the downbeat, '3' being the upbeat or exact middle, and '2' and '4' being the places inbetween or the quarter beats). It is not a synchopated embellishment, as it happens on an even division of the beat.
This tap is demonstrated during back ochos, as well as walking forward and back. Keep in mind that this embellishment can be done during any step with the 'slow' rhythm (stepping just on the downbeat or '1').
Early tango was known as tango criollo, or simply tango. Today, there are many tango dance styles, including Argentine Tango, Uruguayan Tango, Ballroom tango (American and International styles), Finnish tango and vintage tangos. What many consider to be the authentic tango is that closest to that originally danced in Argentina and Uruguay, though other types of tango have developed into mature dances in their own right.