Here we have a classic McCulloch Chain saw. McCulloch made a lot of saws and was an important American manufacturer at one point in history. They produced a number of consumer saws (for weekend woodcutters) and I credit them for being a market leader in addressing saw safety issues relating to kick back. Their saws were among the first I recall seeing with a chain brake, for example.
Kickback is a relatively dangerous condition that arises which can cause the saw to get out of control. It may occur in a couple of ways. First, the top of the chain may grab and push the saw back. If you have a long bar the operator can be forced off balance and literally end up flat on his back with the saw on his belly. The other way is for the bar to hit something and jump. In this case the saw is often pryed out of the operators right hand (which will be on the back of the saw) and the saw pivots on the handle bar implying that the engine goes down and the bar flies up in the operators face.
The chain brake is activated by a lever right in front of the handle bar which is located so that the shifting geometry causes the brake lever to pry against the back of your hand as the saw rotates in your hand. This is suppose to stop the chain before it gets to your face.
This particular saw is not equipped with a chain brake as it is an older model. Note the dogs on the front of the engine. These can be stuck in the log allowing you to lift up on the back of the saw and pivot the bar into the wood.