The mechanical truth
When you sign in, Windows loads C:\Users\<you>\NTUSER.DAT into the registry space as a hive and exposes it at HKEY_CURRENT_USER. Writing HKCU = your account's file updates; nothing else moves. Sign out and the hive unloads; another user signs in and HKCU points at their NTUSER.DAT.
Why this single fact settles most tuning questions
“Will this affect other users?”
HKCU write → never: they have their own hive file
“Do I need admin?”
HKCU write → never: your hive is yours by default ACLs
“Does uninstall of the tool revert it?”
only if the tool journals; Windows has no 'return my hive' function
All the per-user guarantees quoted elsewhere on this blog — the startup blob, the visual values, the privacy bits — live here. Once you see HKCU as a file path rather than an abstraction, scoping stops being philosophy and becomes plumbing you're comfortable reasoning about in the shell.
ProofTune ProjectEngineering notes — every claim here names the bytes a real tool touches. Verify first, install second.