Automation Preview mode lets you audition changes to a mix and compare them to existing automation values without committing them to the automation playlist.
When you put Pro Tools in Automation Preview mode, you can preview values for automation-enabled controls on any track that is in a writable automation mode.
While in Automation Preview mode, when you touch or move a control, it
isolates the control, disengaging it from its automation playlist. The control stops reading or writing automation, allowing you to freely audition changes.
When you have found a level or state that you want to use, you can then commit or
punch the previewed value to the automation playlist for the isolated controls.