Freeing up space · One-tap clean-up (Run Tuhi)
What does the "Reclaim old local backups" step of Run Tuhi do?
Short answer: It thins the oldest local snapshots on your startup disk, preferring ones already on Time Machine and never touching ones macOS requires. Your files and your Time Machine backups are unaffected.
It thins the oldest of the local snapshots macOS keeps on your startup disk — the short-term copies that can hold on to space your clean-ups should have freed. It prefers snapshots that are already saved to your Time Machine destination, and it never touches snapshots macOS requires.
Your files stay exactly where they are, and your Time Machine backups on your backup drive aren't affected. Because macOS already treats this space as available, the summary reports it as reclaimed backups rather than as freed space — see why it's a separate line.