The throttle interlock trigger prevents the throttle from engaging accidentally. If the interlock is not depressed, the throttle cannot engage. The feller must have his left hand firmly on the handle to depress the interlock and trigger at the same time. The throttle interlock prevents the chain from being driven if the trigger is accidentally engaged.
The broad bottom of the handle and the chain catch work together to protect you if the chain breaks. Turn the saw over and take a look at the chain catch. These break off and get lost occasionally. They can be purchased at the saw shop and easily replaced.
If the throttle interlock is not functioning properly, the handle is damaged, or the chain catch is missing, the saw should be immediately locked out of service and tagged for repair. Take the bar off so nobody grabs the saw and loads it up.