Primary muscles worked are the biceps. Secondarily are the forearms.

Position yourself with one knee on the floor and let the back of your arm rest on your inner thigh on the other leg. Grab the single handle with an under hand grip. Keep your arms straight in the starting position. Curl the weight up until you reach full contraction in your biceps. Lower the weight slowly and under control. Repeat with your other arm when you have finished your set.