I have had chains jump out of the bar at times. When a new chain is being used it often heats up and expands. The chain needs tightened if it loosens up to the point you can lift it more than 1/4 inch (when stopped). Another thing, all chain saws come with safety chains today. Just throw it away and buy a professional chain without the anti-kick spurs. The safety chains cannot dig into the wood and are very slow cutting. To me this is as bad as a dull saw…the safety device makes it more dangerous. What gives it away in your case is that the hired man was having to rock the saw to get it to tooth on the wood. Once you understand how a chain saw works and are comfortable with the operation of cutting wood.
I Seriously suggest, if you really want to cut wood, or want to have your saw properly tuned, get out of the home improvement store, and check out a real chain saw / power tool store.
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