Chad just said exactly the same thing you did.  What do you disagree with?

Chad:  "The jake opens the valves and lets out the air on the up stroke as the engine is coasting rather than letting that compressed air push the piston back the other way."

AllSand  "When activated, the Jacobs Engine Brake opens the exhaust valves near the top of the compression stroke, releasing the highly compressed air through the exhaust system.
Little energy is returned to the piston,"