FF has a glossary for the curves explaining each one...
unplug and plug the curves and change them for different effects.
How do you know?... glossary and trial and error... then when you get a feel for them you can make your own curve structures. Experience working with inhouse filters will also help,, as well as filter experience like blade pro and such where you work with bevels. Remember bevels? Those are curves...
I know I just threw an aweful lot at you... it will take awhile to play around with this. Feel free to unplug, add, delete ect to make your own curve structure.
Get some rest and tomorrow I'll play with a neon look for ya (I got carried away on these but thought you might want to play with them and practice curves).