B,
Pick a weight lighter than your normal standing db curl, probably around 60-75%. As you curl it, turn your wrist out (palm out). You should feel a really good pump in both your bicep and forearm.
Another (better, more extensive) explanation I found on the net ...
Using a dumbbell in each hand with the arms hanging in front of the thighs, begin a curl with the right hand with the dumbbell describing an arc with the near end of the dumbbell almost grazing the abs and pec as it nears the right shoulder. At the shoulder turn the wrist so that as the descending half of the circle continues, a reverse curl is being performed as the bell goes away from the right side of your torso and continues to the starting point. BUT. When the bell reached the shoulder, then begin the left hand curl inward grazing the abs and pec on its ascent. Both arms 180 degrees out of phase with each other- as one hand is at a shoulder, the other is at a thigh. Zottman curls are impossible to perform while seated, or on an incline bench.
I think what I do is a little different. Or my explanation is lousy. But there's the real explanation. I read that Zottman was known for his incredible grip strength. Hope this helps.