Always consult the official Owners Manuals and Guides first.
Information about the FM3 may be incomplete or wrong, it's being worked on.
Spillover refers to the functionality to keeping the trails of the delay and/or reverb audible after switching the effect off or switching presets, scenes, channels or X/Y.
All FAS processors support spillover under certain conditions as explained in the manuals.
Spillover between presets
Spillover between presets is enabled or disabled in the Global menu. It requires additional steps to make it work, as explained in the manuals.
The setting in the Global menu applies to switching between presets only. Not to switching scenes, channels or X/Y, or bypassing blocks.
True independent spillover is not supported because it would require double the processing.
"Global spillover settings only apply to preset changes. Spillover between scenes is, in general, desirable and controlled by the bypass mode and state of the various blocks." source
Turn off Spillover in the Global menu if you prefer “silent” switching between presets.
Forum member: "I have two patches that both has delay blocks disabled and when I switch between these patches via MFC the delay spillover sometimes kicks in really loud."
"This can happen if you rapidly switch between presets many times. The unit requires a small amount of time between preset changes to clear unused buffer space. This is done as a background task." source
Spillover between scenes, channels or X/Y
The Global Spillover setting doesn't apply to spillover when turning off effects or switching scenes, channels or X/Y. Delay and Reverb trails always ring out when switching scenes, depending on the effect block's Bypass Mode setting, see below. This is inherent to the way scenes work. When it comes to spillover, scenes work better than presets and are easier to set up.
"Spillover is in there but the "right" way to do it is to use scenes. If you stay within a preset and use channels and scenes the spillover is perfect." source
Spillover can be affected when switching between the X/Y states or Channels of an effect, if drastically different algorithms exist between the two states. For example, if one scene has a Digital Delay and the next scene uses a Tape Delay, spillover will probably not function correctly as these modes use different algorithms.
Scene controllers allow more control over what happens with the sound when changing scenes.
Spillover when bypassing effects
The Bypass Mode of an effect block determines if the reverb or delay trails ring out after turning off the effect, or are muted.
If you use the Mix and Level parameters to dial in the desired effects level, the volume level of the reverb and/or delay trails may change when switching presets. This can be prevented by keeping Mix and Level at fixed values, and using Input Gain to set the desired effects level. source