This is one of the compound exercise involving Smith Machine that power ups your middle back.

Instructions to do this exercise properly

• Initially set the height of barbell on smith machine where it is 2 inches below your knees.
• Bend your waist so that your torso comes forward and your back should stay straight and parallel with floor. Keep your head up.
• Hold the barbell where your palms should face floor (over hand grip). Before that you have to unlock barbell from machine and let the barbell hang in hands.
• While breathing out, lift the barbell where your elbows will go upward and you should feel pressure at your back. Hold this position for a while for better stretch.
• Slowly release this position and go back to starting position, breathe in while doing this.
• Repeat till failure.

For variations you can also use dumbbells in place of barbell though you might not be using smith machine.

Video Demos on how to do this exercise

video 2.