No one exercise will trim your waist and rid you of excess fat. The only way to get a slimmer torso is to lose weight all over, using a combination of cardio exercise, strength training and a healthy, calorie controlled diet. Twists, plank holds and crunches do strengthen your abdominal muscles -- but they won't help you lose the fat that covers them.
The fat that accumulates at your waist is especially receptive to diet and exercise weight-loss strategies. Cardiovascular exercise that gets your heart pumping and your breathing rate faster done for a minimum of 150 minutes per week is a start. Examples include stationary cycling, brisk walking or dance aerobics.
You may have to work your way up to 60 or 90 minutes per day for maximum results, though. Build lean muscle to make your body more efficient at burning calories all day long with two total-body strength-training sessions per week. Cut back on the processed foods, sugary sweets and alcohol. Eat mostly fresh, green vegetables and lean protein, with moderate portions of nuts, fruits and whole grains on the side.