touch off

verb

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.”