Thanks for the detailed explanation. So then the fish seems to actively do better than 'start from scratch' when it falls out of the sweet spot which is the mystery of how.
No problem. As displayed in the video of the dead fish, although initially well-placed and enjoying the neutral flow region/sweet spot, perturbations eventually force it forwards into the obstacle. The live fish has to be able to detect those perturbations effectively enough to avoid similar situations. If not, the fish would have to actively swim back into the sweet spot to begin enjoying the benefits again, as you say 'start from scratch'.
