if you disable a track, the track still appears in your timeline, but the plugins are not loaded and the instrument is not loaded. the workflow basically is, you would render out your track to audio, then disable the instrument track.
Perhaps this is what I don't understand: Would you use this ability in addition to the new render-in-place feature, or instead of it? (I had assumed that the new bounce ability automatically disabled the plugins on that track/channel, but is that not the case?)
the render in place actually uses this functionality, when you render in place you have the option of disabling the instument track which frees up the CPU etc of all the plugins but gives you the ability to quickly go back to the original track if needed.. best of both worlds