Fostaire has a very good anwer.
When flying an Alouette in the Alps many years ago, we were trying to get up to a mountain hut. I was dutifully flying with the ball in the center and thinking that, given the pathetic rate of climb (about 300fpm if memory is still good) that we were never going to get there. The Swiss test pilot suggested that I put the the slip string in the middle instead, and we tripled the rate of climb to just over 1,000fpm.
In all my flight testing, I found the slip ball to be useless below 60 knots - there's just not enough side force for it to react properly.
And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...