The cut-off lever is a small part on the underside of the mechbox that allows the semi-automatic mode to function. It engages the sector gear and trigger assembly to cut off the current from the battery as soon as a semi-auto shot is complete, forcing the user to re-engage the circuit to fire another shot.
The cut-off lever's position is marked.
http://www.airsoftretreat.com/gallery/data/624/medium/17.jpg
=== IIn. The Trigger Assembly: ===