A cutback is the same as a top turn however the turn is continued bringing you around and back towards the curling wave.
You would perform a cutback in order to regain the momentum necessary to continue on the wave. When you are surfing, you may generate so much speed that you find yourself placed well in front of the breaking wave. At this point, the water is flatter (wave has faded, out into deeper water, or not yet formed up) and has less energy resulting in you losing speed/energy.
To regain momentum, you should return to the steeper part of the wave near the breaking curl. If the wave continues to form up in front of you, this eliminates the need for a cutback as you have continual fall and face on the wave to generate speed and momentum.
HOW TO PERFORM A CUTBACK (FRONTSIDE)
Step 1: Generate enough speed on your take off, and begin to eye your path towards the shoulder of the wave
Step 2: Begin a shallow bottom turn and transfer your weight to your back foot and place it near your tail kicker
Step 3: Get your body low and compressed and drive out and away from the curl
Step 4: Transfer your weight from your toeside rail to your heal side rail
Step 5: Rotate your hips, arms, and head around as you extend your body into the turn
Step 6: Light pressure on your front foot and turn your head over your outside shoulder
Step 7: Allow your leading arm to guide you and control the extent of your cutback
X Leaning too hard and far over into your turn without enough speed, resulting in you falling off
X Go too far out in front of the wave and end up in flat water. At this point, you run out of speed to get back to the energy source of the wave (the curl)
X When leaning over the turn, the rail can catch in the water and track (like a railway line). Therefore stopping you from performing the arc of your turn and resulting in you wiping out