While seated on an incline bench and with dumbells
hanging in a palms facing each other position,

curl the dumbells up while slowly rotating them to a
position where the palms are now facing the body.

To
work the bicep further during this rotation, you can
use an offset grip with your thumbs touching the side
of the dumbell.  As with all curling exercises, try to
prevent any movement of the upper arm.

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Many thanks to Donna Slaga for providing the photos and description