Freeing up space · What's Using Space (Storage Inspector)
What does the Force move back toggle do?
Short answer: It lets a move back proceed even while an app is using the folder, with the fast-drive copy winning any conflict. Files that exist only on your Mac are always kept.
It lets a move back go ahead even if an app is using a file in the folder, and makes the copy on your fast drive win on any conflict where both sides changed. It's the amber row at the bottom of the Move back sheet.
Either way, files that exist only on your built-in drive are kept — force only decides which version wins when the same file differs. Use it when you're sure the fast-drive copy is the one you want; otherwise quit the apps and use a normal move back.