The Zercher squat has been one of the biggest secrets of powerlifters over the years. A great movement for building your squat and deadlift because of the development can create for the torso and glutes.
To perform this movement attach the straight bar to the a two sided cable machine. Hold the bar in the crook of the elbow. Pull as much air into your torso and you can, filling you belly with air. You do not want this air in your chest. At this point make your entire body as tight as you can. As you sit down flex your abdominal as hard as you can.
When using a machine like this the movement becomes very unstable to do. Use lighter weight and slow the rep speed down and keep your body tight. Due to the extreme nature of this movement (you body is flexed as hard as you can form start to finish) 5-6 reps per set seem to work best.