Maui Zipline Harnesses basically come in two styles: the older style where you grip the carabineer with your steering hand and the newer style that allows you to sit back in somewhat of a lounge sling and simply enjoy the ride.
The older style, which is typically found on all the original courses in Hawaii, has you sitting in a more upright position. You are strapped in around your waist and thighs and a perpendicular strap which attaches to the trolley wheels comes straight down from the cable above past your sternum to the harness. Your steering hand is used to control a carabineer which allows you to maneuver yourself in either direction, to the right or to the left.
The newer models are a bit more high-tech and to some feel a bit more comfortable. Your position zipping down the line is more prone and tending more toward horizontal to the ground below. Steering is not really a major factor in the newer models.
Keep in mind that no matter what type of harness you use you will ultimately be delivered to the exact point on the exit ramp whether you have the capability to steer or not.