Among the three genres of tango music, milonga is the fastest. Milonga music is 2/4 time. There are two beats in each measure and each quarter note receives a beat, counted as 1-and, 2-and (or 1 +, 2 +; 1 is the downbeat), which is twice as fast as tango. (See Tango Music and Its Danceability.) How well one dances milonga depends on one's ability to handle its fast rhythm.
There are three ways to do that.
When the foot is blocked by an obstacle, it would either cross over to land on the other side of the obstacle, or rebound in the opposite direction. Similarly, there are two ways to do milonga traspie.
The second is rock traspie, resembling the foot rebounds in the opposite direction after meeting the obstacle. In rock traspie, the dancer uses a rock action to transfer weight from one foot to the other foot back and forth three times in quick-quick-slow timing: (1) steps forward/side/back with one leg and transfers 50 percent of the body weight to that leg on the downbeat, (2) pushes with that leg to transfer weight back to the standing leg on the upbeat, (3) pushes with the standing leg to transfer weight completely to the other leg again on the downbeat to free the standing leg.