Cover the steaks with plastic wrap and gently pound them 1/4 inch thick with the flat side of a meat mallet. Combine the rosemary, pepper, and salt, and press it onto the skirt steaks. Coat a large skillet with the oil and set it over high heat. When the oil is just barely smoking, add the meat, lower the heat slightly, and sear the steaks (in batches if necessary) for 2 to 3 minutes on each side for medium rare. The steaks should be very rosy pink. Transfer the meat to a cutting board and tent with foil. With tongs or a spoon, pick any blackened bits out of the pan and discard. Lower the heat to medium and add the vinegar--it will bubble furiously, and the fumes will be sharp at first. Raise the heat and bring the vinegar to a boil. Continue boiling, stirring and rubbing with a wooden spoon to dislodge any brown bits stuck to the pan, until the vinegar is reduced to about 3 tablespoons, about 5 minutes. Stir in any juices collected from the meat. Cut the steaks into 1/4-inch-thick slices. Divide the arugula among four plates and arrange the meat slices on top. Spoon the sauce over the meat and the greens and serve.