Curl from behind for a deep stretch the front can't hit.
Keep the elbow back behind your body to maximize the stretch.
Stepping forward so the cable trails behind you puts the working arm slightly behind your torso, which stretches the long head of the biceps before the curl even starts. Because it is a cable, the tension never disappears at the bottom the way it does with free weights hanging straight down. It earns its place as the stretch-position curl that attacks the range most curls skip entirely.
Starting with the arm behind the torso lengthens the biceps at the bottom of every rep, and loaded stretch is prime growing territory. A normal curl never puts the muscle in that position, this one lives there.
Far enough that the cable is taut and pulling your arm back and down at the start, usually a full stride or more. If there is slack at the bottom, you have thrown away the stretch that makes this curl worth doing.
Both put the arm behind the body to stretch the biceps, but the dumbbell loses tension near the bottom while the cable keeps pulling through the entire range. No bench required either, just the low pulley and a step forward.
