Think of the dumbbell handle as a knife handle and hold it with both hands (it will be a bit small, so imagine overlapping your hands). When you swing it down from above, let it go behind your head. Then, modify it by supporting the upper part of the dumbbell with your palm to get into the starting position. Also, while straightening your elbows, apply strength to your hands to maintain the shape of the supported dumbbell, ensuring it never hits your head.