Any time you need greater control of your bike. With your hads in the drops of the handlebar, you put more weight on your front wheel, which stabilizes the bike and increases your braking power. On long descents, move your hands down before your speed rises above your comfort level. For some new cyclists, riding in the drops can cause hand, neck and shoulder discomfort. Correct handlebar selection and brake-lever adjustment and proper bike fit should allow most people to comfortably ride in this position.
