I would expect that it would turn regen on high while it is running to get the most possible regen. The setting for regen is basically a setting for what happens when you take your foot off the accelerator -- how much will it regen, thus slowing you down. So if you're not putting your foot on the accelerator because of TACC, and letting the car control it, I would assume it uses as much regen as it can whenever it needs to slow down...