The hazard
Machines can start up without warning, and stored energy can be released even after the power is off. Lockout tagout keeps a machine from moving or releasing energy while someone is working on it. It only works when everyone follows the same steps, every time.
The action
Energy can be electrical, mechanical, hydraulic, pneumatic, chemical, thermal or another kind, and one machine can have several sources. Lockout goes on an energy isolating device, like a disconnect switch, a circuit breaker or a line valve. Push buttons, selector switches and other control circuit devices are not energy isolating devices, so hitting the stop button is not lockout. (29 CFR 1910.147(b))
ASK THE CREW
Who on this crew is authorized to lock out, and who is affected?
From the talk: Lockout/tagout (LOTO). Read it in full, free, with the questions and the sign-in sheet.