Hello.
-Envelope follower staff- might be a better description for what this topic discusses.
Anyway, I've been using this method to automate dynamics of Notion LSO Sounds and VSTI's w/rules for a while now. Other people probably have too.
It's a really nice way to get more control of automating and to keep sustained sounds from becoming static. Monotonous.
Using an adjacent staff as an leader/conductor staff allows the ability to jump-in and control long held sustained Notion or VSTI's sounds that are on different staves.

The concept and use of a leader staff is to add only rests on it and attach dynamic markings and hairpins to them. Then using the Instrument Change dialog to route the leader staff so it controls an instrument that's holding a long duration note on a different staff.
After the note-on sound begins adding hairpins to a following rest and dynamic change the hairpin will control the transition on the target staff's instrument.

For large scores only a few leader staves are really needed. Probably could squeeze multi-voice onto a leader. But you don't need to change all the sounds. By applying the Instrument Change and level changes at key points along the score sonorities and their balance can be independently controlled. The motion of long duration harmonies and unison polyphony can be shaped. Definitively changing the blend or balance. Decimal dynamic level changes can really keep section's and division's that are playing long duration held notes from becoming static and looped sounding.
This method really helps playback realism by keeping a perception of forward progress to the music. Easier too in shaping phrases by changing the balance in the mix. It's easier to fit the flow because it's moving to what's more natural in nature.
Also pseudo playback of accents like fz, sfp. etc. are possible.
If or when hairpins and dynamics are desired to be visually correct.to print the original instrument staff some fanegaling and swapping of things can be done. No big deal. The intended benefit though is better playback.

Here's a simple example that's using the Notion LSO Trumpet and Vlns.I and II.
  Nothing new but it's nice to give more motion to Notion. I know, I know. Not funny.
Pretty neat though.



P.s Here is a link to a post of mine that similarly uses hairpins to control MIDI control change messages for synthesizer parameters.
https://forums.presonus.com/viewtopic.php?p=176781#p176781

,Newberry