Here are the most likely interpretations, along with relevant content for each: – A diagnostic tool for engines In automotive and small engine repair, a "tacho torch" could refer to a combination timing light with a built-in tachometer .
A gun-shaped diagnostic tool used on gasoline engines with spark plugs. It has a bright strobe light (torch) that flashes each time the #1 spark plug fires. The built-in tachometer reads engine RPM from the same signal.