Bent the arms and simultaneously, turn the hand outwards (this works the biceps at both functions: bending the arm and supination of the lower arm).
At the end of the movement an extra stimulus can be given by elevating the elbow until horizontal.
Note: The sitting position has a slight advantage for isolation of the biceps as it is harder to cheat by rocking the upper body back and forward.
The standing position is preferred when the exercise is more aimed at a functional level:
Download free biceps exercise sheet: