Dwell meters are no longer used except in older engines that have points and condenser ignition systems. The meter is used to accurately adjust the timing of the engine to the opening of the points in the distributor so the firing of the spark plug occurres at the correct instant for more power and fuel economy.

Present day automobile engine ignition is controlled by a computer and a number of devises that operate automatically. This really isn't as complicated as it sounds. It used to be that the alternator charged a solenoid that discharged when the points closed. The flow of electricity went to the distributor rotor. When it made contact with a distributor wire electricity went to the spark plug and fired the cylinder. The thing won't work if the electrical flow is continuous so the power needs to be turned off for a millisecond. The "dwell" is the number of degrees that the distributor cam can turn before opening or closing(turning off the power) the ignition circuit via the points. The dwell "meter" measures the rotation of the distributor cam in degrees, .015, .019 etc, These are minuscule distances and milliseconds in time. The positive(red) alligator clip goes on the positive pole of the solenoid and the negative (black) on the negative pole. Start the engine. The dwell is computed by the meter based on the discharge from the solenoid. When the points are open the solenoid will not discharge. In later models the dwell adjustment by a set screw on the side of the distributor. Setting dwell in older models is really tedious. It's been a long time, hope this helps.