Preview Mode
After opening a Live device in Max for editing, there are two copies of the device -- the one you are editing and the one inserted into a track in Live. Preview Mode permits you to listen to the device you are editing while Live runs in the background. The copy of the device inserted in the Live track is disabled.
Audio, MIDI, timing, and messages (including Live API commands) are sent between Max and Live in order to make this happen.
Preview Mode is always enabled when you begin to edit a device in Max.
Enabling and Disabling Preview Mode

Uses of Preview Mode
Preview Mode Caveats