Lockout/tagout.

When you're operating on anything dangerous you lock out the machine so nobody turns it on while you're in a dangerous area.

Best example i can think of is like if you had to fix a jam in a wood chipper and actually put your hands inside of it, you would turn it off and put a lock on it, so only you can turn it back on when you get back.

In the above post someone just cut the lock to presumably turn the machine back on while the OP was in a dangerous area for it to turn on.

Edit: reading the thread looks like the machine was broken and they locked it out to keep it turned off until maintenance could be done on it