hard vs soft knee compression

Hi everyone, first post here, hopefully you can help me out a little. I have a fair bit of experience doing amature recording, and am pretty well versed in the use of audio gear, and the physics of audio (the old Yamaha Sound Reinforcement Handbook is my bible), so I don't think this is a total newbie question. I did check through the stickie'd thread on compression to see if this had already been answered, but I didn't see it there. I apologize in advance if this is something you hear once a month!