Here is some info on trap bar squats from Dolzine Fitness Online
Trap Bar Squats
Some people feel this exercise is more like a deadlift than a squat. Still, for those who have already perfected squat form and cannot deal with spinal compression, trap bar squats will allow you to handle heavy loads. Best of all, your palms face your body which is their natural position when standing. You do need to locate a bar that fits your anatomy (they come in various sizes and weights). Trap Bars are also excellent for stiff-legged deadlifts, regular deadlifts, shrugs and shoulder presses.
The real difference between a squat and a deadlift using a trap bar is in the execution. If you use it to deadlift, you would start by picking it up off the floor or a low rack. If used for squatting, start in the standing position, squat down, then return to the standing position.
Extremely detailed information about
trap bar squats (and most other basic exercises) can be found in Stuart McRobert's "The Insider's Tell-All Handbook on Weight-Training
Technique," which can be ordered through http://www.hardgainer.com
This is an excellent book with good photos that should be in everyone's fitness library. On page 69, Stuart mentions trap bar deadlifts as being an excellent squat alternative and refers readers to page 163, where he discusses trap bars and the execution of the exercise in detail.
The problem becomes holding on to very heavy loads for the duration of the set. You can use straps or use an open rack set-up so that you can pick the bar up, take a couple of steps forward or backward, do the number of reps you can, and easily re-rack the bar.