If you for example take a poly sound using OSC 1, and then use OSC 4 for LFO to modulate OSC 1 frequency, then if you for example set OSC 4 to keyfollow to "no" and play two notes - one after the other - and hold both notes, then you perhaps will hear the LFO modulation is'nt in sync for the two notes, but if you set OSC 4 keyfollow to "LFO" then the two notes have the same LFO phase/ synced modulation no matter when you play the notes, - because it will act monophonic as i guess the manual mean.