Insofar as I am concerned, I have never observed this piece of equipment being used correctly.
First, the preacher’s stand is invariably placed too high on the chest. This position allows only the low bicep to work, and Brachialis Anticus, which is a muscle under the low bicep that actually starts the arm to bend, after which the low bicep takes over. This muscle is attached to the forearm and reaches halfway up the upper arm bone. It is responsible for thickness in the low bicep region.
If the preacher’s stand is placed so that the top is 3 inches lower than the low pectoral, you can finish the curl properly, which is to lean forward so the high bicep and the Coracobrachialis comes into play.
The high placement of the preacher’s stand results in a low maximum bicep and Brachialis pull, but no high bicep and Coracobrachialis development. This produces a flat bicep instead of a round look.
To go further, the outer head of the bicep is developed by placing the elbows in and the hands wide. This position can only be successfully obtained by resting the elbows on a preacher’s stand so as to provide a firm placement for the elbows.
The inner head of the bicep is developed by placing the hands close together (3 inches apart) and the elbows 20 inches wide.
Needless to say, the regular hand placement on the bar for the regular preacher’s curl is 11 inches apart, with elbows the same width, which develops the center portion of the bicep.
Another hint is to let the barbell roll down to the end of the fingers and then start your curl. As you close your hand start curling by bending your wrist and flexing your forearms.
After you finish as many reps as your strength permits, step back from the preacher’s stand and curl the barbell 4 or 5 more reps with elbows well back and the bar as close to the body as possible (drag curl), touching the neck at the top of each rep. This type of burn is superior to half reps with elbows on the preacher’s stand, since working extra low bicep and Brachialis you also further work the belly of the bicep, high bicep and Coracobrachialis.
Minimum rest between sets is a must. This is what I call a high tension workout or an honest workout.
Now if you wish to specialize on the biceps, work a maximum of three days a week and pump with lighter poundages two days a week. The common concept of specialization is overwork, and overwork produces muscle tissue loss (catabolism).
The proper number of sets and reps differ with the individual (anywhere from 8 to 15 sets). I personally have never observed anyone grow on more than 15 sets, and as you know, I have trained more champions than any other gym in the world.