If prices rise, short sellers start losing money. In order for them to cut their losses, they will have to buy back the shares they sold short, at a higher price. The key is that in order for them to cut their losses they have to buy back the stock to replace that which they borrowed. If there is a huge short position, it means that there are going to be a lot of new buyers coming back into the market and buying. This drives the stock even higher, and in some cases can be like throwing gasoline on an open fire.
Prices jump hard and fast.
They climb nearly vertical as short sellers panic to cover their positions and cut their losses. By paying attention to the days to cover you can measure the future buying pressure in a particular stock. As a contrarian you can use this information to see which stocks people are betting against. If the news starts turning positive then you will see a lot of buyers entering the market, not only bulls but bears as well who are forced to cut their losses. With stocks that are in good shape technically you like to see the number of shares shorted increase as it indicates rising pessimism.