The standard power twister exercises focus on the chest, forearms and triceps with secondary emphasis on trapezius, rhomboids, and deltoids. There are several ways to directly target the biceps directly, however. You just have to change the position of the twister in relation to your body and the nature of the twisting motion employed. The following article describes several unique bicep exercises that you can perform with just your power twister.
http://www.ehow.co.uk/way_5673495_power-twister-exercise-biceps.html