The following exercise works on your biceps. It's a great isolation exercise because your biceps are doing the work and it's hard to cheat and help using the back or the shoulders.
You should adjust the height of the preacher board such that you can put your armpits at the edge of the boards when comfortly seated.
Then you lift the barbel such that your arms are perpendicular to the ground. That's your starting point.
Then you lower the weight down *IN A CONTROLLED MANNER* until your arms are *almost* fully streched. You should not straighten your arms completely cause it's extremely hard to lift it from there. Also you don't let the weight throw your arms unless you want to rip the biceps tendon from where it inserts on the forearm bone which shall require a surgery and months of inactivity.
As a general rule - don't throw the weights, don't shake your body, don't overload and don't try to impress no one - cause no body cares. Size doesn't matter here. The trick is to work with less weight but in a perfect form.