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').