This is a common embellishment for back ochos and takes a little practice to work up the speed to do it but once you do you'll find lots of places to fit it in. During back ochos, beat your free foot in front as it passes the other on the "&" count (halfway through the beat), before stepping behind into the next step. (You should feel the achilles tendon at the back of the free leg hit the lower part of the shin of the standing leg, just above the ankle.) Try not to let your legs turn-out as you do this and make sure the free foot does not sickle or flop - either point your foot or flex it but make sure it is energized.
The leader will not give you extra time to do this, he will continue moving forward so you have to be fast to get your foot in front of your standing leg and then back out again without tripping yourself up. Think of the accent being 'out!' away from the leg, instead of 'in!' towards yourself. Be careful not to stab yourself with your heel or kick the leader.
Eventually try to get it so the beating movement only happens from the knee down of the free leg and cannot be felt in the upper body. Try not to tense your muscles as you do it and remember to breathe!
At first you may find it easier to beat the left foot in front of the right as you have slightly more space on the open side of the embrace. Once you get good at it try not to beat on every step, but only when you hear an accent in the music.
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.