TheKillerPenguin

It depends on the exercise you're doing with the dumbells. what types of exercises are you doing?

Edit: reading your post again makes me think you mean curling.

the way I do em is sit on my bench, put one leg out at 90* the bench, and the other at 45* with that leg. If I'm curling with my left arm, my right leg is at 90* to the bench, and the opposite is true with my right arm. When I curl, I bring my arm and the weight down slowly at about halfway between my two legs, and bring it back up more quickly up to my shoulder. You want to use a weight that allows you to isolate your arm muscles from the rest of your body; however, this obviously gets harder the more reps you do in the set. I do wind up using my legs and my body to help when my arm tires out. Not sure if it's good form (it probably isn't, actually), but it won't hurt anything unless your back is really weak (and hopefully it isnt since you're a cyclist!)