The grooves around the body of the dart engage the interior bore of the sabot pieces. They look like threads, but in most cases they are grooves. Typically the sabot is cut into 3 or four pieces that circle the dart. The firing gases push the large diameter (full bore) of the sabot. The grooves are just like a buttress thread arranged so that the flat part is on the point side so it pushes the dart forward, but can fall away from centrifugal force and gas pressure once the projo leaves the bore. The forward surface of the sabot is funnel shaped, so it catches the air and opens like a parachute, and the sabot pieces leave the dart. The dart continues on to target. These can't be fired over the heads of troops, as they would get hit by sabot pieces, or from jet aircraft, as the engines would suck in the sabots. Thats why there are a different type of API projos for jet aircraft.