If the shot is too hard, the ball can go over the lip and the back wont stop it in the least.

In addition, when putting downhill it is common for the ball to roll quite a bit further than the golfer planned. The key is a putt that is soft enough to just topple the ball into the hole.

What usually happens is that they decelerate during the through-stroke, tightening their muscles and opening the club face which starts the ball off the target line, even if it catches the lip, it spins violently out of the cup and because it is downhill it rolls to far to be a tap in.

There are two ways taught to avoid the problems of a downhill putt, the first is to strike the ball with the toe of the putter.

Hitting away from the sweet spot (or the best point on the club) it applies less force causing the ball to roll more slowly.

With this method however, you take the chance of opening up the putter face which causes a putt to the right. Gripping the putter a little more tightly keeps the face from opening (turning).

Another means is to choke down on the putter, in some instances actually gripping the shaft rather than the handle. This effectively shortens the club allowing less power on impact.

If the downhill is extreme, dont be afraid to grip almost to the putter face, it is more difficult, but can be done with practice.

After learning and practicing this method, you will find it is easy to hit a short, controllable distance, even on a fast downhill.