With less momentum from the strike on a downhill putt, gravity will act upon the ball sooner on its run towards the hole and force the ball down the direction of the true downslope.
On downhill putts, therefore, we need to allow for more break.
Remember: less speed equals more break.
Uphill putts are much easier than downhill putts because they have less break due to the momentum of the strike required to hit the ball up the hill.
Remember: more speed equals less break.
The ball will take any break when it starts to "die" (loses speed), this is when gravity starts to takes over and the ball will follow the true downslope.