Digital BW, The Print

I typically use the split channels method to render B&W from colour but have recently 
begun playing with the channel mixer.  I guess I need some help as to what the sliders do 
- I am simply fiddling in the dark at the moment.  Should the sum of the Red, Green and 
Blue slider percentages ideally add to 100?  What is the purpose of the Constant slider?  If 
a channel is raised beyond 100% what does this mean? (I found that some quite dramatic 
effects can be produced by say raising the red to 200% and having -50% for the other two 
channels but it seems that skies can get torn up by doing this.)

Thanks

Steve