When Show Advanced Tools is selected in the Advanced preferences pane, you can turn tracks off in the Tracks area. Turning off a track lets you silence a track while still hearing other tracks in the project assigned to the same channel strip.
When no other track is assigned to the same channel strip, turning off a track also saves processing power, because plug-ins on the channel strip are no longer processed. Turning a track off (or turning it back on) takes slightly longer than muting or unmuting the track, due to internal pre-processing. Unlike muting a track, turning off a track can not be automated.
You can also control dynamic loading of plug-ins on a track using the On/Off button. When you open a project, plug-ins on tracks that are turned off aren’t loaded, even if the tracks contain regions. While working on the project, plug-ins aren’t loaded on a track that is turned off when the track is focused, or when regions are added to the track. When the track is turned on, plug-ins are loaded if the track becomes focused. You can also make the plug-ins on a track inactive using the On/Off button.
Plug-ins load dynamically if the "Only load plug-ins needed for project playback” checkbox is selected in Project Settings > General.
Click the On/Off button 
in the track header.
To turn the track on, click the On/Off button again.
Hold down Option and click the On/Off button in the track header.
To reactivate plug-ins on the track, Option-click the On/Off button again.
When a Software Instrument track is turned off, all MIDI regions on the track are silenced, but live input (playing) is still audible.