The most critical part of roasting a whole pig is securely trussing it to the spit. As the meat cooks the muscle fibres and tendons loose their cohesiveness and pull away from the bones. Additionally, the constant rotation of the carcase causes the meat to continually shift position. Together these issues can cause the pig to literally fall apart under its own weight. To prevent this you must truss a pig on a spit so the carcase is not able to move while cooking.
Place the pig on a level work surface. Insert the spit between the hams of the pig's hind quarters. Run the spit along the spine, inside pig's body cavity, through the pig's throat and out of its mouth.
Pull the spit through the pig, with your hands, until the pig is centred on the spit. Grasp the spit just behind the pig's hind quarters and push the spit tightly against the pig's tail end.
Use the shears to cut one 12-inch length of twine for each six inches of the pig's spine, including the neck. For example, if the pig's spine is three feet in length, from tail to head, you will need to cut six 12-inch pieces of twine.
Insert the twine through the eye of the trussing needle. Beginning immediately forward of the pig's hind quarters, insert the needle into the pig's back, from the outside, along the spine. Push the needle into the pig's body cavity. Reach inside the body cavity and pull the needle and twine through. Stitch back up through the pig, along the opposite side of the spine, from the inside out. Remove the needle from the twine and tie the twine in a tight square knot. Cut off the ends of the twine, so it doesn't burn, about 1/8-inch above the knot with the scissors. Repeat the procedure, every six inches, along the entire length of the spine.
Stitch the front and back legs together, along the leg bones, with the needle and twine in the same manner as the spine is stitched to the spit. Cut off the ends of the twine 1/8-inch above the knot.
Tie the ends of the front and back legs together with twine. Cut off the ends of the twine.