A frequent criticism of Time Machine since its invention has been that people don’t necessarily want it to run automatically, because it can sap resources and slow down the Mac.
Well, in macOS Sierra there’s now a switch in the Time Machine section of System Preferences, where automatic backups can be turned off.
Once this is activated you must click the Time Machine icon on the menu bar, and select to backup whenever you feel the need. You’ve always been able to do this, of course, but by turning off automatic backups it becomes the ONLY way to backup via Time Machine.
Notably, turning off automatic backups also turns off the local snapshot feature that temporarily backups files to the Mac’s hard disk when the Time Machine drive (or Time Capsule) isn’t available.
UPDATE: As is pointed out in the comments below turning off automatic backups was in fact possible with earlier releases of macOS (OS X) by simply clicking the switch in System Preferences so that it went from ON to OFF. Despite the switch now reading OFF, which implies Time Machine has been turned off (!), you could actually manually backup by clicking the option on the Time Machine menu bar icon. So what’s really happened with macOS Sierra is that the ambiguity has been removed, rather than there being a new option.