touch off

verb3

1. •
   To start (something); to set off traps; especially used for unstable situations that may magnify if disturbed.

   • Be cautious talking about religion or politics, lest you touch off an argument.
2. •
   To cause a fire to start.

   • A spark touched off the conflagration.
3. •
   To establish a reference coordinate or datum, by moving the tool head to just touch the workpiece or machine bed.

   • A traditional method of touching off uses a piece of thin paper placed between the tool and workpiece.