Although the exercises differ, a common factor of each is lifted up until the forearms are vertical with the elbows and upper arm remaining close to the body.
The fullest range of motion is when the elbows begin in full extension, in a supine grip. The biceps contract to lift the weight upward through an arc, to a point where further movement is not possible. The second part of the motion has the elbow joint extending. The weight is lowered back to the start position.